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If you are involved in the design, specification and/or installation of electrical equipment, machines & control systems, take note of changes that will come into force in BS7671 (18th Edition July 2018). A number of these changes are detailed below.
The link between BS7671, machinery & site safety regulations
Small scale production facilities up to large manufacturing sites must comply with the Electricity at Work Regulations. These Regulations and the HSE refer to BS7671 as a “Code of Practice” which sets out “minimum safety requirements for the installation” as compared, for example, to EN62061, which details safety aspects related to the design of machines. PUWER Reg 8 makes it an offence to install equipment unless the manufacturer has provided the necessary information for its safe use.
UK based OEMs, SIs and Agents or Distributors for imported equipment sold in the UK, have to meet Product Safety Regulations and are required to provide information relating to safe installation and use. This includes information on protective conductor currents (earth leakage currents) and RCD characteristics. For example any equipment which is to be connected on a TT supply, or for connection via industrial plug/socket, must be compatible with any existing upstream RCD characteristics (see clause 531.3.3 below).
BS7671 2018 (Draft BS7671 June 17/IET Web Site) includes changes to existing clauses and additional requirements for RCD protected circuits, with the aim of reducing electrical fires and the risk of electrocution, whilst increasing installation reliability and availability. A number of examples are given below:
531.3.2 Unwanted tripping (new clause)
The installation of new feeders and or division of the installation may be necessary to reduce unwanted operation of RCDs, due to increased protective conductor current, especially when adding additional equipment to the installation. RCDs are designed to trip when the residual current exceeds 0.5 -1 x I?n, therefore this regulation recommends:
“(ii) in order to avoid unwanted tripping by protective conductor currents and/or earth leakage currents, the accumulation of such currents downstream of the RCD shall be not more than 30% of the rated residual operating current.”
This safety margin is critical when considering the increased protective conductor currents and harmonic currents associated with filter stages in modern lighting, power supplies, speed control and other energy saving devices.
Unwanted tripping may occur when loads are switched on and off. The use of transient resistant and short time delayed (10mS) RCDs may be considered as means to reducing this occurrence. For 30mA Type A applications the Type AKV will withstand <3kA fast transient and achieve the requirements for electric shock protection in chapter 41. For 100mA and above Selective RCDs have a similar resistance to tripping.
531.3.3 Types of RCD (new clause)
The type of RCD installed must be capable of providing the required level of protection based on the residual currents that may be generated under possible fault conditions:
“Different types of RCD exist, depending on their behaviour in the presence of DC components and frequencies. The appropriate RCD shall be selected from the following”, this clause goes on to list the various types of RCD namely A, F, B and their suitability for different types of residual current. Type AC RCDs are only listed for general purpose applications e.g. resistive loads that generate sinusoidal residual currents at mains frequency.
The inclusion of this new regulation highlights the requirement in all applications to select and install the correct type of RCD based on the residual current characteristics of the circuit e.g. in the case of an installation containing 3 phase VFD/VSD, the upstream RCD and or RCDs must be Type B, irrespective of the application or location. The internal topology of a Single phase VFD/VSD will determine the residual current characteristics (Type A, F or B) and therefore the type of RCDs that can be installed safely upstream. Annex A53.1 of BS7671 gives details of typical faults currents associated with circuits containing semiconductors and the required RCD type (see example 1 and 2). Section 7 gives additional requirements for special locations/applications.
Installers should refer to the equipment manufacturer’s installation recommendations, relating to protective conductor currents, minimum RCD sensitivity and type, before designing and quoting for new installation work involving any semiconductor based products. On site checks will identify if existing RCDs, protective conductor currents and harmonic levels are compatible with the addition of new equipment.
411.3.2 Maximum disconnection times (revised clause)
The maximum disconnection times given in table 41.1 previously referred to final circuits not exceeding 32A.
Table 41.1 now relates to final circuits < 63A with one or more socket-outlets or fixed equipment <32A.
If the required disconnection time is achieved through the use of RCDs, the type of RCD must be compatible with the equipment that may be connected to the sockets or spur. If Type A RCDs are installed up stream and it is possible that equipment producing Type B leakage currents could be plugged into a socket, the use of a special Type B MI RCD should be considered for use in the socket. These units detect and trip if the smooth DC residual current > 6mA, to protect upstream Type A RCDs. This solution is more cost effective than replacing upstream Type A RCDs with Type B.
These recommendations along with other changes are directly linked to reducing the risks associated with electrical faults, namely fire and electrocution. The clarification provided by the inclusion of Regulation 531.3.3 with regard to using the correct type of RCD based on the characteristics of the connected equipment, will make it easier for the Installer, final users, Fire Insurance Assessors and HSE Inspectors to identify where there are differences in the requirements of this Regulation and the specification/design of the installation and installed equipment.
For additional information on RCDs principles and selection, download Doepke Techpub-16 (www.doepke.co.uk/download/Techpub-16)
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