Reduced Low Voltage Systems

Reduced Low Voltage Systems
22 May 2012, 14:50

The requirement to ensure that the earth fault loop impedance in Reduced Low Voltage (RLV) circuits meets the requirements of BS 7671 is sometimes overlooked.

RLV systems − commonly used on construction sites in the UK − provide a high level of safety if they are correctly installed, tested and maintained. This is because of the relatively low nominal voltage to earth of 55 V in a single-phase system and 63.5 V in a three-phase system, the line to line voltage being 110 V in both cases. Safety is further enhanced by automatic disconnection of the supply in the event of an earth fault, which is an essential requirement for fault protection.

Basic requirements for automatic disconnection
Regulation 411.8.3 of BS 7671 requires that the earth fault loop impedance (Zs) at every point of utilisation, including socket-outlets, in an RLV system is such that automatic disconnection occurs within 5s in the event of an earth fault.

The automatic disconnecting device may be either an overcurrent protective device (fuse or circuit-breaker) in every line conductor or a residual current device (RCD). More detailed information on the maximum permitted values of Zs for both types of device is given later in this article.

 

Use of RCDs should be avoided in RLV systems on construction sites
For reasons explained below, the use of RCDs should be avoided in RLV systems on construction sites. RCDs should be employed in such RLV systems only where it is impracticable to achieve a sufficiently low value of Zs to allow an overcurrent protective device to be used for earth fault protection.

Where an RCD is used on a construction site, this should be done with considerable care. The mechanism of an RCD can be particularly susceptible to the harsh conditions of dust, water, impact and vibration likely to be encountered in such locations. The RCD must therefore be suitably selected and erected to withstand these conditions. For example, the enclosure for the device should be suitable, and careful consideration should be given to its location.

Unwanted operation of an RCD on a circuit supplying temporary lighting on a construction site could easily result in injury to persons. Therefore, the rated residual operating current (IΔn) of the RCD should not be unnecessarily low in relation to the value of earth fault loop impedance (Zs) in the circuit. In this respect, it should never be found necessary to use an RCD with a rated residual operating current IΔn as low as 30mA.

Maximum Zs value for an overcurrent protective device
Where an overcurrent protective device is used as the device for automatic disconnection in an RLV system, Zs must not exceed the maximum value given Table 41.6 of BS 7671, for the types and ratings of fuse and circuit-breaker covered in that table. For other types and ratings of fuse and overcurrent circuit-breaker, the maximum permitted value of Zs is given by the formula in Regulation 411.4.5, which, for an RLV circuit, is:

Where:

  • Zs is the earth fault loop impedance
  • Ia is the current causing the automatic operation of the disconnecting device within 5s.
  • U0 is the nominal voltage to Earth (55 V in a single-phase system and 63.5 V in a three-phase system)

It should be noted that, for a given type and rating of overcurrent protective device, the maximum permitted value of Zs when used in an RLV system is considerably lower than when used in a low voltage system of nominal voltage (U0) 230 V.

For example, for a 10 A Type C circuit-breaker to BS EN 60898, the maximum permitted value of Zs when used in an RLV system is either 0.55 Ω or 0.64 Ω, depending on whether U0 is 55 V or 63.5 V (Table 41.6 of BS 7671 refers). If the same device is used in a low voltage system where U0 is 230 V, the maximum permitted value of Zs is 2.3 Ω (Table 41.3 refers), for a disconnection time of either 0.4s or 5s.

Accordingly, particular care is needed in matters such as keeping the lengths of outgoing circuits from transformers sufficiently short, so that the highest value of Zs for each circuit will within the maximum permitted value of Zs for the overcurrent protective device.

Maximum Zs value for an RCD
As already mentioned, the use of RCDs should be avoided in RLV systems on construction sites, where practicable.

Where an RCD is used as the automatic disconnecting device for an RLV circuit, the product of the rated residual operating current (IΔn) in amperes and Zs in ohms is not to exceed 50 (Regulation 411.8.3 refers). This means that maximum permitted values of Zs for RCDs are generally higher than for overcurrent protective devices.

Table 1 gives the maximum permitted values of Zs for a range of RCDs, according to their rated residual operating currents, based on the condition Zs IΔn ≤ 50.

 

Note: To avoid unwanted tripping, the rated residual operating current (IΔn) of the RCD should not be significantly lower than is necessary to co-ordinate with the value of Zs in the circuit concerned.

Inspection and testing
To verify that the highest value of Zs in each circuit is within the relevant maximum permitted value, and continues to be so, inspection and testing must be carried out both prior to circuits being put into service and periodically thereafter. BS 7671 requires that, where protective measures are used which require knowledge of the earth fault loop impedance, the relevant impedances are to be measured or determined by an alternative method (Regulation 612.9 refers). If there is a need to calculate the earth fault loop impedance at a point in a circuit fed from the secondary of a step-down transformer, guidance is given in IET Guidance Note 5 Protection against Electric Shock.

Particular inspection and testing considerations for construction sites
Work on construction sites tends to be in a constant state of change, and the RLV systems used therefore need to be readily adaptable to suit the ever changing situation. Consequently, RLV systems on construction sites usually comprise a number of purpose made transportable or mobile distribution assemblies complying with BS 4363 and/or BS EN 60439-4, interconnected by flexible cables, cable couplers, and plugs and socket outlets.

Electrical equipment on construction sites is likely to be subjected to adverse environmental conditions and possibly physical abuse. The necessary use of flexible cables and plug in connections to supply moveable plant can introduce problems of maintaining an effective earth return path. If cable runs become excessive, or cables and connections become damaged or are allowed to deteriorate, the earth fault loop impedance may increase and the required disconnection time of 5s may not be achieved.

Furthermore, automatic disconnection cannot occur under earth fault conditions if the circuit protective conductor becomes disconnected or broken. HSE Guidance Note HS(G) 141 Electrical safety on construction sites suggests that moveable installations be formally inspected monthly and that combined inspection and testing be carried out, by a competent person, prior to the moveable installation (or any part of it) being put into service and then at 3 monthly intervals.

The frequency of inspection and testing should, however, be kept under review, using the results of previous inspections and tests to determine whether, on the basis of experience, the frequency needs to be increased or, alternatively, might be reduced. For example, if inspection and testing on several consecutive occasions shows the installation to be safe, it may be possible to reduce the frequency. The results of all tests carried out should be recorded and records maintained, such that any deterioration in the installation can be identified and appropriate safety measures taken.

Further information on maintenance, inspection and testing of construction site installations is given HSE Guidance Note HS(G) 141, and in HSE Guidance Note HS(G) 107 Maintaining portable and transportable electrical equipment

The ESC Essential Guide to the Wiring Regulations is available on subscription at www.esc.org.uk/industry/essential-guide/cc

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