Machines containing Variable Speed Drives associated with RCCBs Complying with essential installation safety requirements!

Small stand-alone machines/equipment containing 3 phase VSDs must not be connected to circuits protected by standard RCDs. Connecting 3 phase inverters down-stream of standard RCDs will affect the prescribed operation of the RCD and therefore does not meet the Electricity at Work Regulations.


EWR 4(1)65.The safety of a system depends upon the proper selection of all the electrical equipment in the system and the proper consideration of the inter-relationship between the individual items of equipment.

RCD Protection with VSDs / Electricity at Work Regulations

Where the precautions taken include an RCCB to reduce the risk of death or injury (Regulation 8): Three phase VSDs must only be used with Type “B” RCCBs. The Duty Holder (Regulation 29) makes reference to the Machine / VSD manufactures recommendations relating to the type and characteristics of the RCCB required.

Understanding the risks

AC operational leakage currents: VSDs and their associated EMC filters and motor cables, generate leakage currents at nominal supply frequency (50Hz) and at various harmonic frequencies. Leakage currents in the higher frequency ranges can be significant and from a safety perspective cannot be ignored. Figure 2 details the frequency range of various leakage currents present in a system containing a 32 amp VSD; at 50Hz the leakage current is less than 3mA , however the actual maximum leakage current occurs at 7815 Hz and is approaching 2000mA. RCD Types AC and A; only detect leakage currents at nominal supply frequency i.e.UK = 50Hz.


DC Residual Currents: Under certain fault conditions 3 phase inverters can generate smooth DC residual currents in the AC supply. DC currents flowing through standard RCDs will result in magnetic saturation of the AC sensing coil, preventing the safe detection of earth leakage currents.

The Electricity at Work Regulations makes reference to the IET Regulations as a guidance document (unless the site is specifically covered by another code of practice such as Mines & Quarries), existing Regulation 331.1 is quite clear in its requirement “An assessment shall be made of any characteristic of equipment likely to have harmful effects upon other electrical equipment”. A simple risk assessment carried out under Regulation 331.1 in accordance with existing Health & Safety legislation, on a system containing VSDs and requiring RCCB protection, will quickly identify that Type B RCCBs must be used with a characteristic that is compatible with the operational and safety requirements of the installation, for People and or Fire protection.


Chaz Andrews – Technical Manager, Doepke UK Ltd

If you would like more detailed information relating to this subject, Doepke UK have a free 60 page Technical Application Guide Available on request, or log on to to download the guide or obtain further information on RCCBs. For technical support please e mail


You may also like...