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Minimizing unnecessary live testing for initial verification

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Initial verification is the process of inspection, testing and certification. This is an important process that is carried out before putting an electrical installation into service. This article focuses on the testing aspect of this important process and looks at the minimum level of live testing required to verify the protective measure, automatic disconnection of supply (ADS), in the event of a fault in an electrical installation, in accordance with BS 7671:2018+A3:2024 (referred to as BS 7671 hereafter).

What is initial verification?

Regulation 641.1 of BS 7671 requires every installation, during erection and on completion, to be inspected and tested before being put into service to verify, so far as is reasonably practicable, that the requirements of BS 7671 have been met.

Regulation 641.4 states that “precautions shall be taken to avoid danger to persons and livestock, and to avoid damage to property and installed equipment, during inspection and testing.”

What are common working practices?

It is often the case that people do things a certain way because they have been taught that way and never questioned it. Many inspectors and testers carry out earth fault loop impedance testing for all circuits, which is not necessary and can be potentially dangerous. The information required for initial verification can often be obtained using dead testing. To be safe and to keep on the right side of the law, live working needs to be minimized. It is important to ask yourself; could some testing be done dead?

What is required by the law?

The Electricity at Work Regulations (EAWR) 1989 apply to all employed and self-employed persons. Every work activity, including operation, use and maintenance of a system, and work near a system, shall be carried out in such a manner as not to give rise, so far as is reasonably practicable, to danger.

Regulation 14 of the EAWR states:

Work on or near live conductors

14. No person shall be engaged in any work activity on or so near any live conductor (other than one suitably covered with insulating material so as to prevent danger) that danger may arise unless–

    1. it is unreasonable in all the circumstances for it to be dead; and
    2. it is reasonable in all the circumstances for him to be at work on or near it while it is live; and
    3. suitable precautions (including where necessary the provision of suitable protective equipment) are taken to prevent injury.

If an accident occurred when carrying out live testing, the question that would be asked in a court of law would be; was it necessary to carry out live testing? For some aspects of initial verification, it is clearly necessary to carry out live tests but not in all instances.

What tests are required to be carried out by BS 7671?

Regulation 643.1 of BS 7671 states that the tests of Regulation 643.2 to 643.11, where relevant, shall be carried out and the results compared with relevant criteria. The words “where relevant” are important as they mean exactly that.

The sequence of tests for initial verification in Figure 1 is extracted from IET Guidance Note 3: Inspection & Testing, 9th Edition. The tests are described in Regulations 643.2 to 643.11 of BS 7671.

Figure 1 Sequence of tests for initial verification

How to determine the characteristics of available supply or supplies?

Regulation 132.2 of BS 7671 states that “information on the characteristics of the available supply or supplies shall be determined by calculation, measurement, enquiry or inspection.” Therefore, it is permitted to determine the external earth fault loop impedance (Ze) and prospective fault current (PFC) by enquiry to the distribution network operator (DNO). However, an enquiry to the DNO is likely to result in being quoted the maximum PFC for a single-phase supply of 16 kA and the typical declared maximum Ze values for supplies up to 100 A, as below in Table 1:

Table 1 Typical maximum external earth fault loop impedance values

System Ze
TN-C-S 0.35 Ω
TN-S 0.8 Ω
TT 21 Ω

 

NOTE 1: In ENA ER P23/1:1991, this value was quoted for both protective multiple earthing (PME) and protective neutral bonding (PNB) earthing arrangements. Higher values may apply where the consumer was supplied from small capacity pole-mounted transformers and/or long lengths of low voltage overhead line.

NOTE 2: The external earth fault loop impedance for TT systems consists of the resistance of the neutral to earth plus the impedance of the transformer winding and line conductor, but does not include the resistance of the consumer's earth electrode.

The DNO, distribution system operator (DSO) or independent distribution network operator (IDNO) are required to provide this information in writing, according to Regulation 27 and Regulation 28 of the Electricity Safety, Quality and Continuity Regulations 2002, as amended (ESQCR), or Regulations 28 and 29 of ESQCR (NI).

In some cases, this is not helpful because the maximum earth fault loop impedance for a device could be lower than the Ze value stated. For example, according to Table 41.3 of BS 7671, the maximum earth fault loop impedance for an 80 A Type C circuit-breaker to BS EN 60898 is 0.27 Ω. This is not helpful as it could lead to designers specifying residual current devices (RCDs) for fault protection unnecessarily.

The other issue with determining Ze by enquiry is that it doesn’t verify that the installation is earthed. The safest way to determine earth fault loop impedance is to measure external earth fault loop impedance once and calculate the earth fault loop impedance values using measured R1 + R2 values.

Regulation 643.7.3.201 of BS 7671 requires the PFC to be measured, calculated or determined by another method, at the origin and at other relevant points in the installation. However, in domestic (household) premises, where a consumer unit that conforms to BS EN 61439-3 is used and the maximum PFC declared by the distributor is 16 kA, it is not necessary to measure or calculate PFC at the origin of the supply. Appendix 14 of BS 7671 provides further information.

For installations other than domestic (household) premises, the PFC can be measured at the same time as carrying out external earth fault loop impedance testing.

What is required to determine earth fault loop impedance?

The most common live test carried out unnecessarily is earth fault loop impedance testing. A value of Zs is required to verify ADS in accordance with Section 411 of BS 7671. Regulation 643.7.3.1 states that “Where protective measures are used which require a knowledge of earth fault loop impedance, the relevant impedances shall be measured, or determined by an alternative method.”

An alternative method of determining Zs is by calculation. To determine the earth fault loop impedance, it is simply a case of adding the measured R1 + R2 value for the circuit to the Ze value. The R1 + R2 value is the resistance of the line conductor (R1) and circuit protective conductor (R2), which are determined by dead testing. This reduces the need to access live terminals multiple times unnecessarily. R1 + R2 testing can also aid in verifying polarity. It would not be considered acceptable to measure the Zs and subtract the Ze value to calculate the R1 + R2 values. This is because the Ze is measured excluding any parallel paths, whereas Zs includes parallel paths and therefore, usually results in lower measured Zs values.

Where overcurrent protective devices such as fuses or circuit-breakers are used, Zs shall be measured or determined by an alternative method, such as calculation as described previously. The results are compared with maximum earth fault loop impedance values from manufacturers’ data or from Table 41.2, Table 41.3 and Table 41.4 of BS 7671. It is important to note that the maximum values listed will need to be adjusted to allow for temperature correction. This is explained in further detail in this IET Wiring Matters article by Craig O’ Neill.

This is to verify that the Zs value is sufficiently low to create an earth fault current high enough to operate the protective device within the maximum disconnection times stated in Table 41.1 of BS 7671.

How to verify automatic disconnection of supply for residual current device protected circuits

Where the protective measure ADS is used, the method of verification depends on the protective device. Where RCDs are used for fault protection, knowledge of earth fault loop impedance is not as important because RCDs to BS EN 61008/61009 are designed to operate within 300 ms at the rated residual current. So, the value of earth fault loop impedance doesn’t affect the disconnection time, but it is important to ensure that the value is low enough to provide an earth fault current sufficient to operate the RCD. For example, the maximum earth fault loop impedance for a 30 mA RCD to conform to Regulation 411.4.4 of BS 7671 for TN systems is 7283 Ω (see the calculation below).

 

Rearranged to determine the maximum permitted Zs:

 

where:

Zs is the impedance in ohms (Ω) of the fault loop comprising:

  • the source
  • the line conductor up to the point of the fault
  • the protective conductor between the point of the fault and the source.

Ia is the current in amperes (A) causing the automatic operation of the protective device within the time specified in Regulation 411.3.2.2, or Regulation 411.3.2.3, of BS 7671. When an RCD is used, this current is the residual operating current providing disconnection in the time specified in Regulation 411.3.2.2, or Regulation 411.3.2.3

U0 is nominal AC RMS or ripple-free DC line voltage to Earth.

Cmin is the minimum voltage factor to take account of voltage variations depending on time and place, changing of transformer taps and other considerations.

NOTE: For a low voltage supply given in accordance with the ESQCR, Cmin is given the value 0.95.

Where RCDs are used for TN systems, Regulation 411.4.204 of BS 7671 states that Table 41.5 may be applied. However, it is important to note that the maximum earth fault loop impedances are derived from a touch voltage of 50 V, not 230 V.

Where RCDs are used for TT systems, Regulation 411.5.3 of BS 7671 applies, which refers to Table 41.5, therefore, RA × IΔn ≤ 50 V.

Figure 2 Maximum earth loop impedance values for TT systems

Where RCDs are used for fault protection, Regulation 643.7 of BS 7671 states that verification shall be made by visual inspection and testing. Visual inspection is required to confirm the RCD is the correct type and rating. Testing is required to verify that the RCD operates within the time stated by the product standard, for example, BS EN 61008 and BS EN 61009.

It is often argued that RCDs are products and should not require testing if they conform to the product standard. After all, BS 7671 does not require testing of other protective devices, such as circuit-breakers, surge protective devices (SPDs) and arc fault detection devices (AFDDs).

However, Regulation 643.7.1 of BS 7671 requires RCD testing where RCDs are used for fault protection or additional protection. Regardless of RCD type, effectiveness is deemed to have been verified where an RCD disconnects within 300 ms for general non-delay type RCDs with an alternating current test at rated residual operating current (IΔn).

Regulation 643.7 also states that:

“Where the effectiveness of the protective measure has been confirmed at a point located downstream of an RCD, the protection of the installation downstream from this point may be proved by confirmation of the continuity of the protective conductors.”

This means that earth fault loop impedance testing is not required downstream of an RCD. Continuity of protective conductors is deemed sufficient to confirm the effectiveness of ADS.

What testing is required?

Each electrical installation is different, and appropriate testing needs to be selected according to the protective measures. For example, it is common for a residual current operated circuit-breaker with integral overcurrent protection (RCBO) to be used on each circuit for a domestic installation. In such cases, the minimum testing to be carried out is:

  • continuity of protective conductor
  • continuity of live conductors for ring final circuits
  • insulation resistance
  • polarity
  • external earth fault impedance
  • polarity of the supply at the origin of the installation
  • RCD testing.

In this case, the only live testing required to be applied is external earth fault loop impedance, polarity of the supply at the origin of the installation and RCD testing. This requires only one live test at source and one RCD test for each RCD. If Zs testing was carried out, this could increase the amount of live testing needed.

RCD testing should be carried out at the outgoing terminals of the RCD. This is to prevent the wiring or connected equipment from influencing the test results. However, it is often argued that it is safer to carry out the testing with a plug-in test lead at a socket-outlet with no other equipment connected in the circuit. This would remove the need for being exposed to live parts.

Regulation 643.6 of BS 7671 requires the polarity of the supply at the origin of the installation to be verified before the installation is energized. This is a test that can only be carried out live. This test can be carried out at the same time as measuring external earth fault loop impedance. The indicators on a multi-function test instrument will verify polarity at the same time as carrying out an earth fault loop impedance test, so it doesn’t need to be a separate test.

What other testing is required in BS 7671?

Regulation 643.9 provides requirements for ‘check of phase sequence’. It states that “for polyphase circuits, it shall be verified that the phase sequence is maintained at all relevant points throughout the installation.” Checking phase sequence is carried out by visual inspection by checking that the line conductors are connected to the appropriate terminals throughout the installation. Phase sequence is a dead test and is often confused with phase rotation which is a live test. BS 7671 does not provide requirements for phase rotation testing. Phase rotation testing may be required in some circumstances, for example, to determine whether a motor will rotate in the correct direction.

Summary

Initial verification can be carried out in a number of ways. It should not be the case of doing it a particular way because that’s the way it has always been done.

Regulation 14 of the EAWR states that no work shall be carried out on or near live conductors unless it is unreasonable for them to be dead. For initial verification, it would be difficult to argue that it is unreasonable for the installation to be dead for certain tests.

Periodic inspection and testing may pose different challenges to initial verification. Either for initial verification or periodic inspection, it is important that a risk assessment is carried out to determine what live testing is to be undertaken. Where live testing is required, suitable precautions shall be taken as required by Regulation 14 of the EAWR.

Acknowledgments

  • Calum Mansell
  • Craig O’Neill
  • Gary Gundry
  • Graham Kenyon
  • Joe Cannon
  • John Peckham
  • Leon Markwell
  • Mark Coles
  • Peter Monfort
  • Roger Lovegrove.