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Drives with RCCBs: the essential safety requirements24 October 2011The Machine Safety Regulations for the OEM and the Electricity at Work Regulations (EWR) for the user have been in force for several decades in the UK, however companies still appear to be ignoring basic safety requirements, which could lead to serious injury to, or even the death of, employees. Chaz Andrews reportsEquipment containing frequency inverters for speed control (VSDs) connected via cable and plug to a mains distribution socket protected by a standard RCCB, may not meet the requirements of the IEE Wiring Regulations. In addition, untrained support staff using electrical appliances connected to sockets on the same distribution circuit may also be put at risk.
Electricity at Work Regulation 4 places a very broad duty of care on the ‘Duty Holder’ with regard to the electrical systems and the components making up that system: Regulation 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.
The Duty Holder is advised to make reference to reputable National/International harmonised standards as well as industry standard codes of practice, including the IEE Wiring Regulations where appropriate, or seek expert advice; quote “a little knowledge is often sufficient to make electrical equipment function but a much higher level of knowledge and experience is usually needed to ensure safety.” Under Regulation 29, lack of knowledge is not a defence in law.
VSDs and RCCB protection
To comply with Regulation 4(1) 65 - where the precautions taken include an RCCB to reduce the risk of death or injury (Regulation 8) - if the RCCB is not designed or suitable for use in circuits that contain VSDs, the safety of the system is compromised and cannot be put safely into service.
It is therefore imperative for the Duty Holder (Regulation 29 – defence in legal cases) to make reference to the VSD manufacturer’s recommendations relating to the type and characteristics of the RCCB required or, if not clearly stated in the operating instruction, obtain the manufacturer’s recommendations in writing. Depending on the topology of the VSD, and certainly with 3-phase units, it would not be safe to use an ac or A Type RCCB. The problem and associated risks are summarised as follows:
AC operational leakage currents: 3-phase inverters used for speed control 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, as can be seen from the following example:
Case study: consider 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. This installation had to be taken out of service by the user, and the drives manufacture contacted to look at ways to reduce the leakage currents to meet fire protection requirements of 300mA for this site. Fortunately for the user, the standard RCCB they tried to use in the installation tripped on start-up due to the inrush current associated with the EMC filter capacitors.
Also it is worth remembering that standard RCCBs are only designed to operate at the nominal supply frequency - for example, 50Hz in the UK. The Duty Holder in this instance did not just bypass the RCCB or fit a time delay unit to try and get around the problem, he had the system checked. Basic electrical test instruments do not give a complete picture of the leakage currents present under normal operation.
DC residual currents: The topology of 3-phase inverters under certain fault conditions will produce a smooth dc residual current, which would not trip a conventional RCCB. In addition this dc current produces magnetic saturation within the ac trip coil, resulting in a change to the designed trip characteristics for sensing ac residual currents flowing through standard RCCBs.
A product applied outside of its original design tolerance cannot be and should not be relied upon to perform a safety function, covered under IEE wiring regulation clause 133.1.3.
Using B Type RCCBs
The Electricity at Work Regulations makes reference to the IEE 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 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 B Type RCCB must be used with a characteristic compatible with the operational and safety requirements of the installation for “People and or Fire” protection.
To safely commission an installation containing a VSD follow the manufacturer’s instructions. Use a residual current analyser (such as DRCA1) to determine the actual maximum level and frequency of the leakage currents present in the installation - leakage current at 50Hz, maximum leakage current and the frequency at which it is generated, and the maximum frequency and leakage current.
These values will change based on the drive status/power on VSD off/VSD ramping up/down, etc. Having this information available will enable the engineer to validate the RCCB selection, reducing the risk of nuisance trips, but more importantly make certain that the RCCB is selected correctly for the safety requirements associated with the installation.
Chaz Andrews is technical manager, Doepke UK.
Doepke UK offers a free 60-page technical application guide for download from its website. Contact Details and Archive...Related Articles...Most Viewed Articles... |
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