NAPIT’s Bill Allan reviews Section 419.
BS 7671:2018 contains a new Section 419 – Provisions where automatic disconnection according to Regulation 411.3.2 is not feasible. In the context of BS 7671, this basically means practicable or doable.
As Regulation 411.3.2 concerns the maximum permitted disconnection times in case of a fault, any situation where automatic disconnection is not feasible is likely to cause some anxiety.
It has been acknowledged in past editions of BS 7671 – and in the present edition – that there are situations where overload and fault current protection can be omitted if unexpected disconnection of a circuit could cause danger or damage (Regulations 433.3.3 and 434.3).
Regulation 433.3.3 gives examples: the exciter circuit of a rotating machine, the supply circuit of a lifting magnet, a circuit supplying a safety service or a circuit supplying medical equipment used for life support in specific medical locations where an IT system is incorporated. This regulation recommends that, in such situations, consideration should be given to the provision of an overload alarm.
Similarly, Regulation 434.3 states that devices for protection against fault current need not be provided in some situations and it also gives examples of such situations, which include those quoted in Regulation 433.3.3.
Regulation 433.3.3 and Regulation 434.3 should be consulted for the full details.
However, Regulations 433.3.3 and 434.3 are different from Section 419, which is not intended for the purpose of omitting a protective device.
Regulation 411.3.2.5
Before considering Section 419, we must first consider Regulation 411.3.2.5 which refers to, and effectively introduces us to, Section 419. The present Regulation 411.3.2.5 is different to the former version and allows for circumstances where:
- the use of an RCD for this purpose is not appropriate
- it is not feasible for an overcurrent protective device to disconnect the supply within the required disconnection time in the event of a fault (as required by Regulation 411.3.2)
Where is it not appropriate for an RCD to be used to comply with the required disconnection times?
In certain non-domestic premises, unexpected disconnection of a supply by an RCD can cause danger or damage or be disruptive to production. This could happen, for example, in a production line or in a computer suite.
Regulation 411.3.3 permits RCDs to be omitted in non-domestic premises where a documented risk assessment determines that the risk to people in the building is sufficiently low that RCD protection is not deemed to be necessary.
The omission of RCDs entails careful consideration of the risks involved. It should involve liaison with those on site who have detailed knowledge of, and who are responsible for, the operations or work activity. The risk assessment must be attached to the electrical certificate and reviewed at appropriate intervals to ascertain that it remains accurate.
Where is it not feasible for an overcurrent protective device to be used to comply with the required disconnection times?
This topic is covered in Section 419.
Regulation 419.1
Regulation 419.1 contains requirements for circumstances where automatic disconnection of the supply within the required disconnection time may not be feasible. Examples of such circumstances are where:
- the source of supply is inherently incapable of delivering sufficient earth fault current with respect to the time/current characteristics of the overcurrent device (as could be the case where a power electronic convertor is used), or
- the required disconnection times cannot be achieved by a protective device (for example, the earth fault loop impedance may be too high) In such circumstances, Regulations 419.2 and 419.3 apply.
Regulation 419.2 – power electronic convertors
Regulation 419.2 contains requirements for installations where a power electronic convertor is used, the nominal voltage of which is greater than 50 V AC or 120 V DC, and where automatic disconnection is not feasible.
A power electronic convertor is the application of solid-state electronics for the control and conversion of electrical power. It may be converting AC to or from DC, or the voltage or frequency – or a combination of these. Devices that are used in this way include:
- switched-mode power supplies
- PV installations
- EV charging installations
- speed control of motors
In the event of a fault between a live conductor and the protective conductor or Earth, the output voltage of the source must be reduced to 50 V AC or 120 V DC or less within the time required for automatic disconnection.
Regulation 419.2 then refers to BS EN 62477-1 which is entitled, Safety requirements for power electronic converter systems and equipment. General.
BS EN 62477-1 is classed as a group safety standard. This is a reference standard to be used by committees in the development of product standards. It covers a wide range of equipment.
It is expected that manufacturers of power electronic convertors will have to comply with BS EN 62477-1 and provide information for installers.
The power electronic convertor used must be one for which the manufacturer gives adequate methods for the initial verification and periodic inspection and testing of the installation.
Regulation 419.3 – Supplementary bonding
Except where Regulation 419.2 applies and where disconnection cannot be achieved within the required time, Regulation 419.3 calls for supplementary protective equipotential bonding to be provided in accordance with Regulation 415.2 in order that the voltage between simultaneously accessible exposed-conductive-parts and/or extraneous-conductive-parts does not exceed 50 V AC or 120 V DC.
Conclusion
Although Section 419 contains provisions where automatic disconnection in the event of a fault is not feasible, Regulation 411.3.2.5 states that disconnection may be required for reasons other than protection against electric shock, such as protection against fire and thermal stresses in equipment.
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