Maintenance Strategies and Work Management

Introduction and Context

Electrical maintenance is the organized combination of technical, administrative, and managerial actions intended to keep electrical equipment in, or restore it to, a condition in which it can perform its required function safely and effectively. In electrical engineering, maintenance is inseparable from safety: inadequate maintenance can produce insulation breakdown, arc flash, electrocution, fire, unplanned outages, environmental release, and damage to connected systems.

Modern maintenance is not simply the repair of failed equipment. It is a lifecycle discipline involving asset selection, risk assessment, inspection, testing, work planning, procurement, competence management, documentation, and continuous improvement. The appropriate strategy depends on the consequence of failure, equipment design, failure mechanisms, operating environment, available technology, legal requirements, and the cost of intervention.

This chapter compares corrective, preventive, predictive, condition-based, and reliability-centered maintenance. It then develops practical methods for maintenance planning, work control, permits, competency assurance, spares management, and computerized maintenance management system (CMMS) workflows.

Learning Objectives

After studying this chapter, the reader should be able to:

1. Distinguish major maintenance strategies and select among them using risk, failure behavior, and lifecycle cost.
2. Explain reliability, maintainability, availability, and useful maintenance performance indicators.
3. Develop an electrical maintenance plan containing tasks, intervals, procedures, resources, permits, and acceptance criteria.
4. Design a safe work-management process from defect identification through closeout and learning.
5. Specify competency, tools, test equipment, and spare-parts requirements.
6. Describe a CMMS workflow and evaluate its contribution and limitations.
7. Apply maintenance principles to an electrical engineering case.

1. Maintenance as an Engineering System

An asset performs a required function within specified operating conditions. A maintenance task is justified only when it protects that function, reduces unacceptable risk, or restores performance. The central questions are:

• What function must the asset perform?
• How can it fail to perform that function?
• What are the causes and consequences of each failure?
• Can a technically feasible and economically reasonable task manage the failure?
• What must be done if no effective scheduled task exists?

Maintenance should therefore be integrated with operations, electrical safety, engineering, procurement, stores, and management of change. A technically excellent task performed under an invalid permit or by an unqualified person is not effective maintenance.

Important terminology

Failure is the termination of an item's ability to perform a required function. Fault is an abnormal condition that may or may not have caused failure. Defect is a condition that can degrade performance or create risk. Inspection is examination for conformity or deterioration. Test is a controlled activity used to determine performance against specified criteria. Overhaul is extensive examination and restoration intended to extend service life. Backlog is authorized work not yet completed. Outage is a planned or unplanned loss of service. Criticality expresses the consequence and, often, likelihood significance of failure.

2. Comparing Maintenance Strategies

2.1 Corrective maintenance

Corrective maintenance restores an asset after a defect or failure has been detected. It may be:

• Run-to-failure, where failure is intentionally accepted before repair; or
• Deferred corrective maintenance, where a known defect is recorded and repaired later under controlled conditions.

Run-to-failure is reasonable for non-critical, inexpensive, readily replaceable items whose failure creates little safety, environmental, quality, or production impact. It is unsuitable for protection relays, emergency systems, fire pumps, critical switchgear, or equipment whose failure could expose personnel to danger.

Advantages include low planned intervention and simple administration. Disadvantages include emergency labor, secondary damage, production interruption, unsafe pressure to restore service, and uncertain parts availability.

2.2 Preventive maintenance

Preventive maintenance is performed at predetermined intervals or according to prescribed criteria, regardless of the immediate measured condition. Examples include tightening inspections, cleaning, lubrication where appropriate, battery checks, protective-device testing, and scheduled replacement.

Time-directed replacement is effective when failure probability increases substantially with age, or when a statutory test interval is specified. It is less effective when failures are random or when intrusive maintenance introduces new defects.

Preventive tasks should not be retained merely because they have always been performed. Each task needs a purpose, a measurable outcome, an interval rationale, and an acceptance criterion.

2.3 Predictive maintenance

Predictive maintenance uses measurements, trends, models, or diagnostic technologies to forecast degradation and plan intervention before functional failure. Electrical examples include:

• infrared thermography for abnormal temperature patterns;
• insulation-resistance and polarization-index testing;
• partial-discharge monitoring;
• transformer dissolved-gas analysis;
• motor-current signature analysis;
• vibration analysis for rotating electrical machines;
• battery impedance and conductance testing; and
• power-quality and harmonic monitoring.

Prediction is not the same as measurement. A measurement becomes predictive only when its relationship to failure is understood, data quality is controlled, and decision thresholds are defined.

2.4 Condition-based maintenance

Condition-based maintenance initiates work when observed condition indicates that action is required. It can be online or offline, continuous or periodic, and qualitative or quantitative. A thermal image showing a developing high-resistance connection may trigger a planned outage. A low battery capacity result may trigger replacement.

Condition-based maintenance is often broader than predictive maintenance. Prediction estimates future failure time; condition-based maintenance uses condition evidence to decide whether and when to intervene. The two approaches frequently overlap.

A robust condition-monitoring program defines:

1. the failure mode being detected;
2. the physical indicator;
3. the measurement method and accuracy;
4. the normal or baseline condition;
5. alert and action limits;
6. the response time; and
7. the person authorized to interpret the result.

2.5 Reliability-centered maintenance

Reliability-centered maintenance (RCM) is a structured process for determining the maintenance requirements of physical assets in their operating context. It begins with functions and performance standards, identifies functional failures and failure modes, evaluates consequences, and selects technically feasible tasks.

Typical RCM consequence categories are safety/environmental, operational, non-operational, and hidden failure. A hidden failure is not evident during normal operation, such as failure of a standby protection channel. RCM may select condition-based maintenance, scheduled restoration, scheduled discard, failure-finding tasks, redesign, or deliberate run-to-failure.

RCM is not a synonym for “more preventive maintenance.” Its value lies in selecting the least burdensome strategy that manages the relevant failure consequence. It is especially useful for complex systems, critical protection, standby equipment, and assets with multiple interacting failure modes.

2.6 Strategy comparison

No strategy is universally superior. A practical program normally combines all five.

3. Reliability, Maintainability, and Availability

Reliability is the probability that an item performs its required function for a stated time under stated conditions. For a constant failure rate and an exponential lifetime model:

where R(t) is reliability at time t, λ is the constant failure rate in failures per unit time, and e is the base of natural logarithms. The model assumes statistically independent failures, a constant failure rate, and no relevant aging effect. These assumptions are often unsuitable for equipment with wear-out or infant-mortality behavior.

For this model, mean time between failures is:

MTBF is meaningful for repairable equipment under a defined operating regime; it is not a guaranteed operating duration and should not be interpreted as a replacement interval.

Maintainability describes the probability that failed equipment can be restored within a stated time under specified conditions. Availability is the proportion of time an asset is capable of performing its function. A simplified steady-state relationship is:

where A is inherent availability, MTBF is mean time between failures, and MTTR is mean time to repair. The formula assumes statistically stable repairable operation and includes only the modeled failure and repair times. Operational availability may also include logistics, waiting, testing, and administrative delays.

Worked example: availability improvement

A repairable motor-driven pump has an MTBF of 1,200 h and an MTTR of 8 h. Its simplified availability is:

If improved diagnostics reduce MTTR to 5 h while MTBF remains unchanged:

The improvement is approximately 0.25 percentage points. This illustrates that better access, tools, diagnosis, and spares can improve availability even without changing the failure rate.

Maintenance performance indicators

Useful indicators include planned work percentage, schedule compliance, emergency-work percentage, corrective-maintenance backlog age, repeat-failure rate, mean time to repair, safety observations, and maintenance cost per operating hour. Indicators must be interpreted together. A high schedule-compliance value can be misleading if low-priority tasks are scheduled while urgent defects are excluded from the schedule.

4. Developing an Electrical Maintenance Plan

A maintenance plan converts asset requirements into controlled work. It should be based on an asset register, drawings, operating context, risk assessment, manufacturer information, applicable law, technical standards, and organizational experience.

4.1 Asset hierarchy and criticality

The asset register should identify site, system, equipment, maintainable item, unique identification, rating, location, function, manufacturer, model, serial number, and safety information. A hierarchy might be:

Site → Substation → Switchboard → Circuit breaker → Operating mechanism.

Criticality assessment should consider personnel safety, arc-flash exposure, fire, environmental effects, production, service continuity, regulatory obligations, repair time, redundancy, and spare availability. A simple qualitative matrix may classify assets as low, medium, high, or critical. A numerical risk-priority approach can use:

where RPN is a risk-priority number, S is severity, O is occurrence likelihood, and D is difficulty of detecting the failure. This product is a screening aid, not a universal risk model. The rating scales must be defined, and catastrophic safety consequences should not be allowed to disappear into an arithmetic average.

4.2 Task specification

Each task should state:

• asset and exact location;
• function or failure mode addressed;
• task type and interval;
• required isolation and permit;
• hazards and controls;
• competence and supervision requirements;
• tools, test instruments, and calibration status;
• spare parts and consumables;
• step-by-step method;
• test conditions and acceptance criteria;
• restoration and functional checks;
• records and photographs required; and
• escalation requirements for abnormal findings.

Intervals should be based on failure behavior, manufacturer guidance, legal requirements, service experience, and monitoring results. A shorter interval is not automatically safer: excessive intrusive work can increase exposure and human error.

4.3 Maintenance procedure structure

A safe electrical procedure normally includes purpose, scope, references, definitions, responsibilities, prerequisites, risk assessment, isolation procedure, test equipment, work steps, hold points, acceptance criteria, restoration, emergency actions, and records.

A procedure must distinguish de-energized work from energized work. De-energization should include identification of all sources, isolation, securing against re-energization, proving the tester, testing for absence of voltage, and re-proving the tester. The exact sequence and terminology must follow the organization’s electrical safety rules and applicable jurisdiction. Energized work requires a specific justification, risk assessment, suitable equipment, approach boundaries, trained personnel, and an approved method; it must never be treated as a shortcut for poor planning.

5. Work Management and Permit Control

A controlled workflow prevents defects from being lost, unsafe work from starting, and valuable technical knowledge from remaining undocumented.

5.1 Work identification and screening

Work enters through inspection findings, operator reports, alarms, predictive-monitoring results, statutory programs, engineering changes, or failure reports. A planner or supervisor screens the request for validity, duplication, urgency, safety significance, and operational impact. Immediate hazards require prompt protective action, not merely a low-priority work order.

5.2 Planning and scheduling

Planning determines the work scope, sequence, labor hours, access, isolation, tools, permits, drawings, parts, vendors, and test requirements. Scheduling assigns a realistic date and coordinates operations, outages, contractors, and production. The schedule should protect a frozen short-term window while retaining flexibility for emergent safety work.

5.3 Permit-to-work interface

A permit-to-work is a formal communication and authorization system. It does not replace risk assessment or isolation. Typical controls include:

1. define the job and equipment boundary;
2. identify hazards and energy sources;
3. establish and verify isolation;
4. issue the permit to an authorized person;
5. conduct a pre-job briefing;
6. control access and simultaneous operations;
7. suspend or cancel the permit when conditions change; and
8. inspect, clear personnel, remove tools, restore systems, and close the permit.

The permit issuer, isolating authority, performing authority, and work party should have clearly separated responsibilities where required by the safety management system. Permit language should be unambiguous, and shift handovers should be documented.

5.4 Execution, testing, and closeout

During execution, the supervisor confirms the work boundary, controls changes, and stops work when conditions differ from the approved plan. Test results must be recorded with instrument identification, calibration status, environmental conditions where relevant, measured values, and acceptance criteria. “Tested satisfactory” is rarely sufficient evidence.

Closeout includes equipment status, temporary modifications removed, guards replaced, labels restored, protection settings verified, permits closed, parts consumed, labor recorded, defects generated, and lessons captured. Failure reports should distinguish immediate cause, contributing causes, and underlying management-system causes.

6. Competence, Human Factors, and Safety Governance

Electrical maintenance competence includes technical knowledge, practical skill, hazard recognition, procedural discipline, communication, and judgment. Competence is task-specific: an experienced mechanical technician is not automatically competent to test medium-voltage switchgear.

A competence system should define:

• role profiles and minimum qualifications;
• supervised practical experience;
• authorization boundaries;
• assessment methods;
• refresher and reauthorization intervals;
• contractor verification; and
• records of training, observation, and performance.

Human factors deserve explicit treatment. Common contributors to incidents include ambiguous equipment identification, poor lighting, time pressure, fatigue, inadequate drawings, normalization of deviations, and ineffective shift handover. Good design uses clear labels, independent verification for critical steps, hold points, peer checks, stop-work authority, and procedures written for actual field conditions.

Management must ensure that production objectives do not reward unsafe bypassing of permits, testing, or isolation. Safety rules should be compatible with applicable national legislation and recognized standards, such as the jurisdiction’s electrical installation, occupational safety, and electrical safe-work requirements. Organizations must verify the current legally applicable editions rather than relying on a textbook list.

7. Tools, Test Equipment, and Spares Strategy

Test instruments are safety-critical assets. A maintenance organization should control selection, unique identification, calibration, inspection, battery condition, leads, fuses, environmental rating, and storage. Instruments must be suitable for the circuit category, voltage, fault environment, and test method. Calibration does not prove that an instrument is appropriate for a particular measurement; suitability and pre-use inspection remain necessary.

Spares classification

Spares can be classified as:

• routine consumables, such as fuses, terminals, labels, and cleaning materials;
• insurance spares, whose failure consequence is high and replacement lead time long;
• repairable spares, exchanged and refurbished under controlled quality requirements;
• rotable spares, moved between assets; and
• obsolete or strategic spares, retained because replacement equipment or support is difficult to obtain.

Stock decisions should consider failure rate, lead time, supplier reliability, commonality, storage life, obsolescence, repair capability, and consequence of stockout. Battery systems, electronic modules, seals, and some insulating materials may have storage-life limits. Critical protection relays and breaker mechanisms may require controlled environmental storage and configuration records.

Duplicate part numbers and uncontrolled substitutions create serious risk. A substitute must be technically assessed for ratings, fault duty, insulation, dimensions, protection coordination, software or firmware compatibility, certification, and manufacturer approval where required. Stores records should link each part to compatible assets and approved alternatives.

8. CMMS Workflow and Data Quality

A CMMS connects asset information, work orders, preventive programs, labor, materials, history, costs, and reports. A typical workflow is:

Request → Screen → Prioritize → Plan → Approve → Schedule → Issue permit/isolation → Execute → Test → Close → Review and improve.

The CMMS should preserve the distinction between an observation, a work request, a work order, and a completed technical record. Mandatory fields may include asset identification, failure code, cause code, remedy code, labor, parts, downtime, test results, and follow-up recommendation.

CMMS benefits include visibility of backlog, automated due dates, traceability, analysis of repeat failures, parts reservations, and evidence for audits. It cannot compensate for incorrect asset hierarchies, poor procedures, false completion, missing failure codes, or weak field supervision. Data governance should define ownership, naming conventions, change control, access rights, retention, backup, and periodic quality audits.

Mobile devices, sensors, and analytics can reduce transcription and improve early detection. Artificial intelligence may assist anomaly detection, job-plan drafting, image classification, or spare forecasting, but outputs require engineering validation. AI should not independently authorize isolation, override protection, approve energized work, or replace competent diagnosis. Bias, cybersecurity, explainability, data provenance, and cyber-physical consequences must be managed.

9. Illustrative Case: Critical Manufacturing Substation

Background

A manufacturing plant operates a 11 kV/415 V substation supplying process lines, safety systems, and a firewater pump. The substation has two transformers and a normally open bus tie. A CMMS exists, but records are incomplete and maintenance is mostly time-based.

Problem

The plant experiences two nuisance trips in six months. One trip is traced to a loose low-voltage termination. Thermography was performed irregularly, infrared images were not linked to asset numbers, and no action threshold was defined. The second event causes a production interruption because the required breaker mechanism spare is unavailable.

Analysis

The loose termination is a detectable degradation mechanism suitable for condition-based inspection. The breaker mechanism is a long-lead insurance spare candidate. The protection system and firewater supply have high safety and operational consequence, while ordinary lighting circuits have low consequence. The existing program treats all tasks similarly and therefore misallocates effort.

Approach

The plant team:

1. rebuilds the asset hierarchy and identifies critical functions;
2. performs an RCM-style functional and failure-mode review;
3. establishes thermography routes, baseline images, temperature comparison rules, and action limits approved by an electrical engineer;
4. introduces scheduled de-energized torque and connection checks where technically justified;
5. creates failure-finding tests for standby protection and firewater supply;
6. purchases and preserves an approved breaker mechanism spare;
7. revises permits and switching instructions;
8. verifies technician competence in medium-voltage isolation and testing; and
9. makes CMMS fields for defect severity, thermal image, test result, cause, and corrective action mandatory.

Outcome

The revised system changes maintenance from uniform time-based work to a risk-based combination of condition monitoring, preventive testing, corrective repair, and strategic spares. The plant gains earlier warning of connection defects, shorter recovery time for breaker failure, and better evidence that safety-critical systems have been tested.

Learning points

The case demonstrates that maintenance effectiveness depends on the whole work system. A camera, a CMMS, or a spare part has limited value without defined decisions, competence, safe access, quality data, and management follow-through.

10. Challenges, Limitations, and Best Practices

Maintenance optimization faces uncertainty. Failure data may be sparse, manufacturer intervals may be generic, condition indicators may be affected by load and temperature, and different technicians may make inconsistent judgments. Aging infrastructure can contain undocumented modifications. Digital systems introduce cybersecurity and data-integrity risks.

Best practice is to:

• begin with safety and required function, not with a favorite technology;
• use RCM principles for critical or complex systems;
• combine failure history with engineering judgment and monitoring evidence;
• write measurable tasks with explicit acceptance criteria;
• eliminate work that has no defensible purpose;
• protect planned work from unnecessary disruption;
• maintain accurate asset and spare-part records;
• treat permits, isolation, and testing as engineered controls;
• investigate repeat failures and bad actors;
• audit field quality, not merely paperwork completion; and
• review the program after failures, modifications, incidents, and new operating conditions.

Chapter Summary

Electrical maintenance is a lifecycle work system whose purpose is to preserve required function while controlling safety, operational, environmental, and financial risk. Corrective maintenance is appropriate for selected low-consequence items; preventive maintenance uses predetermined intervals; predictive maintenance forecasts degradation from data; condition-based maintenance acts on measured condition; and reliability-centered maintenance selects tasks through function, failure-mode, and consequence analysis.

Reliability, maintainability, and availability provide useful engineering concepts, but formulas require stated assumptions and should not replace field evidence. A sound maintenance plan identifies assets and criticality, specifies tasks and intervals, defines hazards and acceptance criteria, and integrates permits, competence, tools, spares, and records. Work management must control the entire path from request to safe execution, testing, restoration, and learning.

CMMS technology improves traceability and analysis only when data, procedures, responsibilities, and field practices are reliable. Contemporary sensors and AI can support diagnosis and planning, but competent people remain accountable for safety-critical decisions. Effective maintenance is therefore both a technical discipline and a governance system.

Key Terms

Reliability: Probability that an asset performs its required function for a stated time under stated conditions.

Maintainability: Probability that failed equipment can be restored within a stated time under specified conditions.

Availability: Proportion of time an asset is capable of performing its required function.

Corrective maintenance: Maintenance performed to restore equipment after a defect or failure.

Preventive maintenance: Predetermined maintenance based on time, cycles, usage, or prescribed criteria.

Predictive maintenance: Maintenance planning supported by forecasts derived from condition data or models.

Condition-based maintenance: Maintenance initiated when measured condition indicates that action is required.

Reliability-centered maintenance: Structured analysis that selects maintenance requirements from functions, failure modes, and consequences.

Failure mode: The specific manner in which an asset can fail.

Failure consequence: The effect of a failure on safety, environment, operation, or other functions.

Criticality: Relative significance of an asset or failure based on consequence and risk.

MTBF: Mean time between failures for a repairable item under defined conditions.

MTTR: Mean time to repair, usually representing the average restoration time under defined conditions.

Permit-to-work: Formal authorization and communication system for controlling hazardous work.

Isolation: Separation and securing of energy sources to prevent unintended energization.

Competence: Demonstrated combination of knowledge, skill, experience, and judgment for a task.

Insurance spare: Spare retained primarily to protect against high-consequence failure and long replacement lead time.

CMMS: Computerized maintenance management system used to manage assets, work, materials, and records.

Backlog: Authorized work that remains incomplete.

Acceptance criterion: Defined requirement against which inspection or test results are judged.

Review Questions

Short Answer Questions

1. What is the principal difference between predictive and condition-based maintenance?
2. Identify two situations in which run-to-failure may be justified.
3. What information should be included in a maintenance task specification?
4. Why is a permit-to-work not a substitute for isolation?
5. State two limitations of MTBF as a maintenance decision rule.

Long Answer Questions

1. Compare corrective, preventive, predictive, condition-based, and reliability-centered maintenance, including suitable electrical applications and limitations.
2. Explain how an electrical maintenance plan should be developed from an asset register and criticality assessment.
3. Describe a complete work-management workflow from defect identification through CMMS closeout.
4. Discuss the role of competence, human factors, and permit systems in preventing electrical maintenance incidents.
5. Explain how a spares strategy should address criticality, lead time, obsolescence, storage, and approved substitution.

Analytical / Application Questions

1. A critical transformer has frequent thermal alarms but no immediate functional failure. Design a condition-monitoring and decision process, including measurements, thresholds, escalation, and safe work controls.
2. An organization reports 98% preventive-maintenance schedule compliance but suffers repeated emergency failures. Analyze at least five possible reasons and propose performance measures that would provide a more balanced assessment.
3. Using the availability relationship A = MTBF ÷ (MTBF + MTTR), compare an asset with MTBF 900 h and MTTR 10 h with one having MTBF 700 h and MTTR 3 h. Interpret which has higher availability and explain why availability alone may not determine the preferred asset.

Multiple Choice Questions with Answers

Question 1

Which maintenance strategy selects tasks by analyzing functions, failure modes, and consequences?

A. Corrective maintenance

B. Reliability-centered maintenance

C. Run-to-failure maintenance

D. Unplanned emergency maintenance

Correct Answer: B. Reliability-centered maintenance

Question 2

For a constant failure rate λ, which expression represents reliability at time t?

A. R(t) = λt

B. R(t) = 1 − e^(−λt)

C. R(t) = e^(−λt)

D. R(t) = t ÷ λ

Correct Answer: C. R(t) = e^(−λt)

Question 3

A system has MTBF = 1,000 h and MTTR = 10 h. Using the simplified availability formula, its availability is approximately:

A. 90.91%

B. 98.01%

C. 99.01%

D. 99.90%

Correct Answer: C. 99.01%

Question 4

Which is the best example of condition-based electrical maintenance?

A. Replacing every breaker after exactly five years

B. Repairing a motor only after it fails

C. Inspecting a thermal image and repairing a connection when its condition exceeds the action limit

D. Ordering identical parts without inspecting equipment

Correct Answer: C. Inspecting a thermal image and repairing a connection when its condition exceeds the action limit

Question 5

What is the primary purpose of a permit-to-work system?

A. To replace technical training

B. To authorize and communicate controlled hazardous work

C. To eliminate the need for isolation

D. To guarantee that equipment cannot fail

Correct Answer: B. To authorize and communicate controlled hazardous work

Question 6

Which item is most characteristic of a high-quality maintenance work order closeout?

A. The statement “work completed” with no measurements

B. Deletion of the original defect report

C. Recorded test results, equipment status, parts, labor, and follow-up defects

D. Automatic closure at the scheduled date

Correct Answer: C. Recorded test results, equipment status, parts, labor, and follow-up defects

Question 7

Why might an insurance spare be held in stores?

A. It is always the cheapest component

B. Its failure consequence is high and replacement lead time is long

C. It requires no identification or preservation

D. It eliminates all preventive maintenance

Correct Answer: B. Its failure consequence is high and replacement lead time is long

Question 8

Which statement about MTBF is correct?

A. It is a guaranteed operating lifetime for every asset

B. It is meaningful only for non-repairable items

C. It is an average measure under defined conditions and is not automatically a replacement interval

D. It is equal to MTTR for a reliable system

Correct Answer: C. It is an average measure under defined conditions and is not automatically a replacement interval

Question 9

A CMMS is most effective when:

A. All work is closed automatically

B. Asset data, failure codes, procedures, and field records are accurate

C. Permits are removed from the workflow

D. Only equipment cost is recorded

Correct Answer: B. Asset data, failure codes, procedures, and field records are accurate

Question 10

Which action best demonstrates safe de-energized electrical work?

A. Relying on an equipment label without testing

B. Opening a breaker and immediately touching conductors

C. Identifying sources, isolating and securing them, proving the tester, testing for absence of voltage, and re-proving the tester

D. Working energized because the outage was inconvenient

Correct Answer: C. Identifying sources, isolating and securing them, proving the tester, testing for absence of voltage, and re-proving the tester