UPS design impacts 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!

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?

Regulation 551.4.3.3 Additional Requirements for installations incorporating static convertors

The risks associated with static converters / UPS installation are covered by the above regulation, including additional earthing requirements – 551. and the impact of d.c. currents on protection devices (RCD’s) see Clause 551.4.3.2. “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”.

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 d.c. leakage currents. 

For example 3 phase multi-pulse topology will produce leakage currents with a d.c. component that in the case of non-isolated UPS, will flow in the PE conductor under fault conditions. This also applies to the d.c. content produced by smoothing capacitors and EMC filters, present during normal operation.  In this instance from aspect of safety it must be considered that the UPS would be incompatible with up-stream RCDs designed to operate with standard AC supplies see fig 1

Fig 1:  6 Pulse bridge produces a smooth dc leakage current


For the purpose of RCD selection Type A would not be suitable for the above application if the smooth d.c. content exceeds 6mA. Many 3 phase UPS will produce smooth dc currents in excess of 6mA requiring the use of Type B RCD.

Type A: (EN61008 & EN61009) Designed to operate with residual sinusoidal alternating currents (50Hz) and pulsating direct currents with <6 mA smooth d.c. 

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.

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. Installation manuals for UPS products should contain guidance for the installer on the type of RCD required, to meet the Electrical Safety Products Directive.

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


Chaz Andrews – Technical Manager, Doepke UK Ltd  mail:  or