Hi Once, MANY years ago, we used some non-shocksafe US-style fuseholders on an item which was designed for the 115V-0-115V system for the US market - but were running it on a UK 240V supply with one end earthed as is normal in the UK. Unfortunately I forgot that the fuseholders were not shocksafe and pulled the Neutral-end fuse to check it and, of course, was connected to the supply from the 240V end - the shock threw me half-way across the room! Even more unfortunate, two UL engineering inspectors were standing watching at the time! Very painful and embarrassing! :-( This reinforces the need for both over-current and shock-protection on both hot leads with this method of connection - and, if you use fuseholders, ensure that they are the shocksafe type!! John Allen (sorry, no responses for a very long time, but I have been very busy at work - and not getting shocks!)
From: [email protected] [mailto:[email protected]] On Behalf Of Robert Johnson Sent: 25 April 2008 16:24 To: Nick Williams Cc: EMC-PSTC Subject: Re: Earthing of generators Obviously it is safe to connect it as you mentioned 115V-0-115V. The entire US does it that way. HOWEVER, there are expectations. Both lines must be treated as live phases and have appropriate overcurrent protection where needed. When a wire is treated as a neutral, overcurrent protection is usually omitted in that side of the circuit, mostly because ground faults will not cause overcurrents. In the UK where polarized plugs are used and an identified neutral side is present, products can be designed to treat the neutral differently with respect to ground fault protection (see IEC 60950). This is not common and most appliances rely on the branch circuit for ground fault protection. In the US, there is a special code provision for circuits wired 60V-0-60V for use with audio equipment (with the intent of reducing AC hum). This is similar to the condition you propose. The code (NFPA70, 647) requires you to treat both sides as phase conductors and to have two pole common trip overcurrent protection. As a result, you and any customers are unlikely to encounter safety problems with such a connection, but if I were offering the product I would suggest providing access to the bonding point so the equipment could be wired either center earth, as in North America, or end earth, as in Europe. The use of coupled two pole breakers and RCDs (Residual Current Devices also called GFCIs Ground Fault Circuit Interrupters in North America) is also important. I am not deeply familiar with UK wiring regulations, so I have not attempted to include what may be legally required, but only the electrical reasoning. Bob Johnson ITE Safety <http://www.itesafety.com> Nick Williams wrote: I'm looking for some help in identifying some specific requirements for the earthing of small generators (engine driven, say 5 - 10 kVA). For reasons which I'm not entirely clear about, the client wishes to sell a 230V generator with the earth connection made to a mid point on the generator windings instead of one end, i.e. with respect to ground the output will be 115V - 0V - 115V instead of being 0V - 230 which would be normal. I'm trying to find good reasons to say why this is not permitted. Clearly the UK supply Regulations do not permit such an arrangement, but I don't think these would apply to a small generator. I have also identified that BS 7375 (Distribution of electricity on building and construction sites) says that a generated supply should be treated in the same way as a utility supply, and BS 7430 (Code of practice for earthing) recommends the use of an earthed neutral for 230V or three phase 400V generators. However, neither of these constitute what I would consider to be an unequivocal 'thou shalt not do it'. HSE publish a guide on electrical safety on construction sites but this essentially refers back to BS BS 7375 and BS 7430 for detailed guidance. >From a safety point of view, the generator will be fitted with a residual current breaker and since in practice few small portable generators are ever actually properly grounded, it seems to me that the main safety concern is that it's not normal practice to do it this way and it only becomes dangerous when the generator is connected to a network designed with neutral point earthing. This may be sufficient reason to say it's not a sensible proposal, but if it is then I need to be clear on the point, not simply say it can't be done for un-specified 'safety' reasons. Can anyone else offer any insight into (a) any specific requirements which may govern this practice and (b) and reasons why it's actually unsafe? Thanks Nick. - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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