Are you complying with essential safety requirements when you install machines containing variable speed drives (VSDs) associated with RCCBs? Chaz Andrews of Doepke explains.

Equipment containing variable speed drives (VSDs) connected via a cable and plug to a mains distribution socket protected by a standard RCCB, will not meet the Electricity at Work Regulations, and untrained support staff using electrical appliances connected to sockets on the same distribution circuit may also be put at risk.

RCCB protection with VSDs

When the precautions taken include an RCCB to reduce the risk of death or injury (Regulation 8), 3-phase VSDs must only be used with Type B RCCBs. The Duty Holder (Regulation 29) makes reference to the VSD manufactures recommendations relating to the type and characteristics of the RCCB required, or if not clearly stated in the operating instruction, obtain the manufactures recommendations in writing.

Understanding the risks

AC operational leakage currents: 3-phase inverters used for speed control and the 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, as can be seen from the following example. Figure 1 shows the frequency range of various leakage currents present in a system containing a 32A VSD – at 50Hz the leakage current is less than 3mA, however, the actual maximum leakage current occurs at 7,815Hz and is approaching 2,000mA.

DC residual currents: The topology of a 3-phase inverter under certain fault conditions will produce a smooth DC residual current. DC currents produce magnetic saturation within the AC trip coil, preventing the sensing of AC residual currents flowing through standard RCCBs.

The Electricity at Work Regulations makes reference to the IEE Regulations as a guidance document – 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 you cannot use a conventional RCCB, and that a Type B RCCB must be used with a characteristic compatible with the operational and safety requirements of the installation for people and or fire protection.


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