AndyStobbs
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Can any of the electrical installers explain why it catagoricly states in the instructions that the metal cooker hood is not to be earthed? Seems counter intuitive to me?
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Sidewinder
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The manufacturers are plonkers?
BS7671 requires an earth at every point.
More info would really be required for a definitive answer.
BS7671 requires an earth at every point.
More info would really be required for a definitive answer.
brightspark
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they only have a 2 core cable fitted as there double insulated. but every point has to be earthed weather it is used or not so if its fed from a 13 amp fused spur up to that point will be earthed but the cooker hood wont. heres an example http://www.lightingcompany.co.uk/ceiling-lights-c3/double-insulated-lights-class11-c31the reason being they have plastic lampholders not metal
BrokenBiker
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^^ wot he said. double insulated appliances should have no possible way where any metal part that you can touch can give current a path into you or anyone else, hence the lack of earth cable as it isn't needed.
AndyStobbs
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Isn't that backwards, the idea is the Earth rail removes the potential from faulty electrical devices? I understand double insulsted and all that, but couldn't see why a metal surface shouldn't be earthed for good measure. Think I'm with sidewinder.
AndyStobbs
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The two core wire runs against the metal, there is a possibility it could rub through given time and vibration.
rtbcomp
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As said, an earth fault elsewhere could cause an earthed hood to become live. There would have to be a faulty appliance putting a live out to an earth that has become isolated from Mother Earth. Fairly unlikely, but there was a report on the radio a couple of weeks ago where a woman got electrocuted when she touched a bath tap that had become live due to a faulty heater in the bedroom.
AndyStobbs
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So does that mean convention is moving sway from cross bonding?
brightspark
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the silly season for bonding everything metal in sight has ended thank god .u used to have to bond all metal tables and rails and sinks together in industrial kitchens
and the cables are double insulated as well so its hardly likely
brightspark
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providing u have rcd protection at source the main services are bonded eg gas water and oil supply no other bonding is required the silly season on bonding is now not nesesary thank god
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Can you recall which edition that was in? 15th?
brightspark
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it possibly was. time flies
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the snooper
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if the appliance is double insulated, it therfore cant introduce potential to the steel case in the first place, so to add an earth cable to it increases the likely hood of the case becoming live from 0% to 100% should a earthing fault develop elsewhere
AndyStobbs
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Understood, can totally see that logic with a device with plastic cases. Think. I would feel safer knowing a metal object was earthed - just incase a fault did develop in that object. It would be very very very unlucky for a fault to develop inside an object and also have a poor earthing circuit at the same time. But seems less unlucky for an un-earthed device develop a fault and the first thing you would know about it was as it ran down your arm.
the snooper
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you seem to be missing the point, double insulated appliances are designed so that if a fault did devolop everything is insulated then encased in plastic so no way of it becoming live unless someone tampered with it purposly with the aim of causing injury
the snooper
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video players, dvds, stereo's and all manner of appliences fall into the above catergory
Sidewinder
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No they aren't, they are insulated & sheathed.
The outer covering of say a 2 core flex, does not have to meet electrical insulation requirements for the cable to meet the product standard.
Whilst it may well do, it does not have to, thus, an insulated & sheathed cable must not be classed as double insulated.
If you look in the cable manufacturing standards, you will find this to be the case.
rtbcomp
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But as I said, it did happen and someone was killed. It was a rented property and the landlord could possibly have prevented it by doing regular safety checks.
I never understood the idea of bonding everything, the concept of having everything at the same potential is OK, but the rules never considered what would happen under fault conditions. Older houses that had earthing via the water mains were particularly vulnerable when old cast iron water mains were replaced with plastic, and they were less likely to have modern RCD protected consumer units.
