Electrical equipment test requirements before energisation
Author: Viacheslav Yurdyk, quality engineer at LK Energy Group.
Acceptance of equipment at the factory (quality control department) and technical supervision on electrical installation sites.
Short answer
Before being put into service, equipment is tested under chapter 1.8 of PUE — these are acceptance test standards. Thereafter, for the whole service life, the standards in annexes 1 and 2 to PTEES apply — these are in-service tests. The two must not be confused: the same measurement has different permissible values and different intervals depending on which stage you are at.
Chapter 1.8 of PUE covers the main and auxiliary electrical equipment of installations rated up to 750 kV that is newly commissioned or returned to service after technical re-equipment or replacement of component parts [п. 1.8.1]. In other words, it applies not only to new build but after every major replacement on an existing site.
Two documents, two moments
| PUE, chapter 1.8 | PTEES, annexes 1 and 2 | |
|---|---|---|
| When | before commissioning: a new site, technical re-equipment, replacement of component parts | the whole service life after commissioning |
| What it sets | the normative values and scope of acceptance tests [п. 1.8.3] | the scope and intervals of in-service tests [ПТЕЕС, п. 13.1 глави 13 розділу VII] |
| Who it applies to | installations up to 750 kV | consumers operating installations up to 150 kV (above that — a separate document) |
| Types of check | acceptance tests | K — during overhaul, P — during routine repair, M — between repairs |
🔑 The practical point for the client. When a contractor says «we have tested everything», the right follow-up is under which document. An acceptance test report does not replace the schedule of in-service tests, and vice versa.
What is checked in switchgear assemblies
This is our own field, so let us go into detail. Acceptance test standards for indoor switchgear (KRU) and outdoor switchgear (KRUN) [пп. 1.8.137–1.8.140]:
1. Insulation resistance - primary circuits — with a megohmmeter at 2.5 kV, the value being at least 300 MΩ; - secondary circuits — as per clause 1.8.191 (table below).
2. Power-frequency withstand voltage test
Test voltage for KRU and KRUN cubicles [таблиця 1.8.22]:
| Nominal voltage of the switchgear, kV | With porcelain insulation | With insulation of solid organic materials | With reduced insulation level |
|---|---|---|---|
| 6 | 32 kV | 28.8 kV | 18.0 kV |
| 10 | 42 kV | 37.8 kV | 25.2 kV |
| 15 | 55 kV | 49.5 kV | 34.2 kV |
| 20 | 65 kV | 58.5 kV | 45.0 kV |
| 35 | 95 kV | 85.5 kV | 72.0 kV |
The test voltage is applied for 1 minute. The secondary circuits of the cubicles are tested at 1 kV for 1 minute.
🔑 A detail worth knowing as a client: the test is recommended to be carried out before the power cables are connected, with the cubicles assembled and the withdrawable parts in the service position (except withdrawable parts carrying voltage transformers, valve-type arresters and surge arresters) [п. 1.8.138]. So it is not «at the very end, once everything is together» but a stage in the works schedule.
3. Checking threaded contact joints [п. 1.8.139] - sample check of bolt tightening — on 2–3 % of joints; - 🔴 in equipment earthing circuits ALL threaded joints are checked — no sampling; - measurement of the DC resistance of plug-in and threaded contacts.
4. Mechanical tests [п. 1.8.140] - five cycles of racking the withdrawable parts out and in, with a check of the condition and alignment of the plug-in contacts and of the operation of the shutters, interlocks, latches and mechanical parts of the arc protection; - a check of the operation and condition of the earthing switch contacts.
For disconnectors the operation of the device is checked separately: five «close» operations and five «open» operations, and for remotely controlled devices the same at rated voltage on the terminals of the control motors [п. 1.8.135]. And the interlock: it must not permit the main blades to be operated while the earthing blades are closed, or the other way round [п. 1.8.136].
Insulation resistance of devices, secondary circuits and wiring
[таблиця 1.8.34]
| What is tested | Megohmmeter voltage | Minimum permissible value |
|---|---|---|
| Secondary circuits of control, protection and measurement | 1.0; 2.5 kV | 10.0 MΩ |
| DC and voltage busbars on the control board (with the circuits disconnected); each connection of secondary circuits and of supply circuits to circuit-breaker and disconnector drives | 1.0; 2.5 kV | 0.5 MΩ — in installations up to 1000 V; 1.0 MΩ — above 1000 V |
| Secondary circuits containing devices with microelectronic components, for a working voltage up to 30 V / 30–60 V / above 60 V | 0.1 / 0.25 / 0.5 kV | 1.0 MΩ |
| Power and lighting wiring | 1.0 kV | 0.5 MΩ |
| Secondary circuits of switchgear, boards and busbar trunking | 1.0; 2.5 kV | 0.5 MΩ |
Three notes from the same table that often get lost: - measurements are made on all connected devices — drive coils, contactors, instrument relays, secondary windings of current and voltage transformers; - the insulation resistance of wiring is measured with the fuse links removed, on the section between fuses or beyond the last fuses — between any conductor and earth, and also between two conductors; and the loads must be switched off; - in switchgear the insulation resistance of the secondary circuits is measured for each section.
🔴 Why «0.5 MΩ» is the most popular and the most dangerous figure on the internet. It really is in the table — but only for power and lighting wiring and for the secondary circuits of switchgear. For secondary circuits of control, protection and measurement the limit is twenty times higher — 10 MΩ. So «we have 0.6 MΩ, all good» is a statement that means nothing without naming the row of the table.
Why the PUE and PTEES limits for secondary circuits do not match
A straight answer is needed here, because this is what people argue about.
- PUE, acceptance test standards: secondary circuits of switchgear, boards and busbar trunking — at least 0.5 MΩ [таблиця 1.8.34];
- PTEES, in-service standards for KRU and KRUN: insulation resistance of secondary circuits — at least 1 MΩ, with a megohmmeter at 1000 V [додаток 1, таблиця 21].
This is neither an error nor a contradiction — these are different documents for different moments, with a different breakdown into rows. The practical conclusion is single: the test report must state under which document and against which row of the table the measurement was made. Without that statement a report does not answer the question «within limits or not».
And one more rule that settles half the arguments [ПТЕЕС, п. 13.3 глави 13 розділу VII]: results are compared not only against the limit but against the initial data — the values in the manufacturer's passports and test reports, or the results obtained after an overhaul. That is what the passport is kept for throughout the life of the equipment.
Conditions under which testing may be carried out at all
Two requirements that sometimes force the works to be rescheduled — and that is normal:
- measurement of insulation characteristics, insulation withstand voltage tests and sampling of transformer oil are carried out at a positive temperature of not below 5 °C, except in specially stipulated cases [ПТЕЕС, п. 13.6 глави 13 розділу VII];
- for switchgear, the insulation resistance of the primary circuits is measured at an ambient temperature not below 5 °C [ПТЕЕС, додаток 1, таблиця 21].
🔑 What this means on site. «They came to take measurements in February in an unheated chamber» is a reason to reschedule the works, not to argue about the result. A report obtained at a sub-zero temperature will be challenged by the first competent engineer.
Who may carry out testing and what belongs in the report
The right to test [ПТЕЕС, п. 13.5 глави 13 розділу VII]: tests and measurements are carried out by accredited laboratories under programmes (procedures) approved by the consumer's manager, in accordance with the Law of Ukraine «On Metrology and Metrological Activity».
The result: is recorded in test reports or corresponding certificates, which are kept together with the equipment passports [ibid.].
What a client should look for in a report to make it useful rather than decorative:
- under which document the measurements were made — PUE ch. 1.8 or PTEES annex 1/2 — and against which row of the table;
- what was used — the type of instrument and the megohmmeter voltage (0.1 / 1.0 / 2.5 kV — the limit depends on it);
- the conditions — the ambient temperature at the time of measurement;
- the values obtained alongside the normative ones, not merely the conclusion «complies»;
- a comparison with the manufacturer's initial data, if this is not the first measurement;
- who carried it out — the laboratory and its accreditation.
Point 4 is the most important. A report stating only «within limits» does not allow the trend to be tracked: insulation degrades gradually, and the value of a measurement lies precisely in the comparison with the previous one.
What happens if a result falls outside the limits
The Rules answer this directly [ПТЕЕС, п. 13.3 глави 13 розділу VII]: electrical equipment whose measured parameters do not meet the standards must be taken out of service. The procedure and timing of withdrawal are decided by the person responsible for the electrical facilities.
And a separate rule for reading the standards, worth knowing: limits worded «not less than» are minimum values, «not more than» are maximum values; all numerical values «from» and «to» are inclusive of the stated figures [ibid.].
Frequently asked questions
Which tests are required before an installation is commissioned?
The scope and normative values of acceptance tests are set by chapter 1.8 of PUE. It covers the main and auxiliary equipment of installations up to 750 kV that is newly commissioned or returned to service after technical re-equipment or replacement of component parts [п. 1.8.1]. The specific list depends on the type of equipment: for switchgear it is insulation resistance, withstand voltage testing, checking of contact joints and mechanical tests.
How do acceptance tests differ from in-service tests?
By the moment and by the document. Acceptance tests come before commissioning, under chapter 1.8 of PUE. In-service tests run for the whole service life, to the scope and intervals in annexes 1 and 2 to PTEES [ПТЕЕС, п. 13.1 глави 13 розділу VII]. Neither replaces the other.
What insulation resistance is required?
It depends on what exactly is being measured. For secondary circuits of control, protection and measurement — at least 10 MΩ; for power and lighting wiring — 0.5 MΩ; for secondary circuits of switchgear, boards and busbar trunking — 0.5 MΩ; for the primary circuits of switchgear cubicles — 300 MΩ [таблиці 1.8.34 і п. 1.8.137]. This is why a figure must always be quoted together with the row of the table.
Why do different sources give different limits for secondary circuits — 0.5 and 1 MΩ?
Because they are different documents. PUE (acceptance standards) gives 0.5 MΩ for the secondary circuits of switchgear, PTEES (in-service standards for switchgear) gives 1 MΩ. So the report must state which document the measurement was made under, otherwise the value cannot be assessed.
At what voltage is insulation resistance measured?
For the primary circuits of switchgear — with a megohmmeter at 2.5 kV; for secondary control and protection circuits — 1.0 or 2.5 kV; for circuits with microelectronic components — 0.1, 0.25 or 0.5 kV depending on the working voltage; for power and lighting wiring — 1.0 kV [таблиця 1.8.34]. The megohmmeter voltage is a mandatory item in the report.
At what voltage are 10 kV cubicles tested?
Per table 1.8.22: 42 kV for cubicles with porcelain insulation, 37.8 kV — with insulation of solid organic materials, 25.2 kV — with a reduced insulation level. The test voltage is applied for 1 minute. For 6 kV the figures are 32 / 28.8 / 18.0 kV respectively.
Can testing be done in winter?
Measurement of insulation characteristics, withstand voltage testing and oil sampling are carried out at a positive temperature not below 5 °C [ПТЕЕС, п. 13.6 глави 13 розділу VII]. For the insulation resistance of switchgear primary circuits the same limit applies [ПТЕЕС, додаток 1, таблиця 21]. So a sub-zero temperature is a reason to reschedule the works, not to dispute the result afterwards.
Who is entitled to carry out testing?
An accredited laboratory — under programmes (procedures) approved by the consumer's manager, in compliance with the Law «On Metrology and Metrological Activity». The results are recorded in test reports or certificates kept together with the equipment passports [ПТЕЕС, п. 13.5 глави 13 розділу VII].
What must a test report contain?
As a minimum: under which document and row of the table the measurement was made, what it was made with (instrument type and megohmmeter voltage), at what temperature, the values obtained alongside the normative ones, a comparison with the manufacturer's initial data (if this is not the first measurement) and who carried it out. A report with nothing but the conclusion «complies» does not allow the trend to be tracked.
Why compare with the passport if there is a standard?
Because the standard shows «it can still be operated», whereas comparison with the initial data shows the trend. The Rules require exactly that comparison with the initial values — from the manufacturer's passports and test reports, or from the results after an overhaul [ПТЕЕС, п. 13.3 глави 13 розділу VII]. Insulation degrades gradually, and a fall from 300 to 100 MΩ is a signal even if the 300 limit is formally not yet breached in another row.
What should be done if a result is below the limit?
Equipment whose parameters do not meet the standards must be taken out of service; the procedure and timing of withdrawal are decided by the person responsible for the electrical facilities [ПТЕЕС, п. 13.3 глави 13 розділу VII].
How many times is a withdrawable part racked out during testing?
Five times — with a check of the condition and alignment of the plug-in contacts and of the operation of the shutters, interlocks, latches and mechanical parts of the arc protection [п. 1.8.140]. For disconnectors — five «close» and five «open» operations [п. 1.8.135].
How many bolted joints are checked for tightness?
On a sample basis, 2–3 % of joints. But in equipment earthing circuits all threaded contact joints are checked [п. 1.8.139].
Are tests required after equipment is replaced on an existing site?
Yes. Chapter 1.8 of PUE covers not only new construction but also equipment returned to service after technical re-equipment or replacement of component parts of existing facilities [п. 1.8.1].
Testing is part of our job
Every product we build passes quality-control inspection and testing in our own accredited electrical laboratory, and the test reports are handed over with the product. If you need testing or a survey on site, send us a completed data sheet.
This material is provided for information only and is neither an offer nor legal advice. We do not publish the full text of the Rules — only quotations with clause numbers; obtain the official text from the source named above.
See also
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