The existing edition of the IET Regulations (Regs) introduces a number of revisions relating to the selection of specific types of RCD based on the characteristics of the loads / devices incorporated within the installation.

The UPS topology and the internal design of the UPS product will impact on the selection of suitable protection means to reduce the risk of electrocution and or fire. Installing UPS systems without taking into account the impact on existing protection devices and existing earthing arrangements can place People and Property at risk!

The standard BS EN 62477-1:2012 Safety requirements for power electronic converter systems (PECS) and equipment; gives guidance for requirements to reduce risks associated with fire, electric shock, thermal energy and mechanical hazards associated with PECS covered by product standards such as those for standalone UPS systems.   

All modern UPS products contain converters employing solid state switching devices, pF correction and EMC filter circuits which produce a wide spectrum of leakage currents with modified wave forms and at various frequencies, including dc  leakage currents.  To reduce cost and improve efficiency, there are a number of UPS products on the market that do not incorporate safety isolation transformers as a standard feature, so how does this impact on the RCD?

For the purpose of RCD selection it’s possible to consider Type A or Type B as safe for use in installations containing UPS, type AC  are only suitable for use in applications which are limited to residual sinusoidal alternating currents at mains frequency.

Type A: (EN61008 & EN61009) Designed to operate with residual sinusoidal alternating currents and pulsating direct currents with <6 mA smooth DC. 

Type B: (EN 62423) Applications where in addition to the above; high frequency AC  leakage due to capacitive loads and smooth DC residual currents are present under normal operation and fault conditions.

For example a 3 phase Six-pulse topology will produce leakage currents with a dc component that in the case of non-isolated UPS, will flow in the PE conductor under fault conditions. In this instance from aspect of safety it must be considered that the UPS would be incompatible with A type RCDs see fig 1

Although the existing Regs do not specifically mention UPS , the particular risk associated with converters is identified in the Regs see Clause 551.4.3.3. “Precautions shall be taken or equipment shall be selected so that the correct operation of protective devices is not impaired by d.c. currents generated by a static converter or by the presence of filters” .    

These factors combined, make it essential to refer back to UPS Manufactures specification and installation documentation, when making a safety assessment on the impact of the specific leakage currents that will be present under normal operation and under fault conditions – see Regs: chapter 33 clause 331.1. This article does not pertain to cover safety issues related to UPS installation,  but is specifically to raise awareness for further investigation.

For a 62 page comprehensive detailed explanation on the application of ‘B’ type RCDs in circuits and installations containing power electronics please click on the http://www.doepke.co.uk/rcd/Designer.html or visit our web site www.doepke.co.uk.