So why can't my cooker hood be Earthed?

[rtbcomp]

The two core wire runs against the metal, there is a possibility it could rub through given time and vibration.

I think the biggest risk in a cooker hood would be fire caused by a chip pan going up, that could possibly cause a live to casing short.

 

[MattF]

video players, dvds, stereo's and all manner of appliences fall into the above catergory

Something fairly catastrophic would have to happen for the case to become live, though judging from the way half of the crap is built these days....

Personally, I'm with Andy on this one. Metalwork gets grounded on everything barring AV equipment etc. If one can't rely on something as simple as an earth being available and sound, there's something wrong which needs sorting anyhow.

 

[BrokenBiker]

I agree, metalwork should be grounded, if there is a chance that it can gain any potential. however, earthing a double insulated appliance, as stated by someone above, actually increases the chance of the appliance gaining potential and causing harm. if, for instance you don't have rcd protection and do have an live - earth fault that isn't causing enough of a draw to trip the mcb, then your metal cooker hood has a 240V supply, which will try and earth itself through you (path of least resistance) if and when you come into contact with it (to turn the fan/light on/off). if it has no earth lead, then the only possible casue of danger would be if there is a fire to melt the insulation of the leads (which would mean it should be checked for safety before next use, common sense), or if it is badly installed where damage to the cable has occurred (if being hard wired to a spur, which most are iirc, haven't done one for a while) in which case this should be uncovered during the basic visual checks for the installation before it is allowed to be powered. then there are insulation resistance checks to be done before the unit can be passed of as safe to apply power, these show any bond at all between live and neutral, so the chance of you getting a buzz from a certified and tested appliance that meets double insulated standards are beyond low. now that's not to say it doesn't happen, I saw a sparky in work get a buzz of a laptop that had been stripped and repaired, when put back together a cable got lodged between the casing and a screw cut the insulation, making that screw live. luckily the rcd stepped in as it should and all was well after the initial swearing and throwing of laptop charger, but, that was an isolated case where negligence had caused the fault.

if you are that worried about it, find someone with an insulation resistance meter, disconnect the hood and then test between casing and cable. it will show >2Mohm resistance (If using the same megger as me) which means in its current state it is impossible for the casing to reach any potential at all

 

[rtbcomp]

.......If one can't rely on something as simple as an earth being available and sound, there's something wrong which needs sorting anyhow.

That's the crux of the matter, there's no reason why earths should less prone to faults than any other part of the system.

 

[the snooper]

If one can't rely on something as simple as an earth being available and sound, there's something wrong which needs sorting anyhow.

you may as well say, if you cant rely on your tv never going wrong there's something wrong which needs sorting anyhow

faults do happen and by adding an earth cable where one is not required you are adding a risk that need not be there and don't forget the earth is connected directly to the case where as anything double insulated has no chance of coming into contact with the case

 

[skotl]

Here is an excellent debate about whether every metal surface should be earthed, or not. Ironically, it centres around a question of whether the OP should earth his metal workbench, which he uses for welding.

http://www.diynot.com/forums/electrics/earthing-a-metal-clad-workbench.407294/

The majority (but not all) of professional sparks who commented was that the bench should not be earthed, because there is a greater risk of it being a path to ground if someone is using another faulty piece of equipment. I suspect the same applies to the cooker hood - if it is double insulated then it simply does not need insulating (and there is zero chance of the cables fraying / rubbing or being cut by the metal hood, otherwise it couldn't be called double-insulated). It therefore presents a pretty large surface area which could give you a route to ground if you touch it while in contact with a faulty device.

Bear in mind that there could be an undetected fault because most kitchen appliances are double insulated (for the same reasons as above), and kitchen floors will typically be made of non-conducting material. So you have one hand on a damp worktop, and there's a leak into a socket below the worktop (for the washing machine, perhaps) and you reach up to turn on the extractor. Fzzzzztttt...

On a side note, RCDs do save lives but even a tiny current right across the heart (one hand, across the chest, to the other hand) can stop it.

 

[AndyStobbs]

I don't think I would Earth a metal bench, simply because it isn't an electrical item in itself. I can see with that scenario its a deffinite increase in the chances of a shock.

 

[the snooper]

I don't think I would Earth a metal bench, simply because it isn't an electrical item in itself. I can see with that scenario its a deffinite increase in the chances of a shock.

hmm so what happens if you put your double insulated angle grinder on the steel bench and the grinder develops a fault ?

 

[rtbcomp]

hmm so what happens if you put your double insulated angle grinder on the steel bench and the grinder develops a fault ?

Pretty much the same as if you were holding the grinder.

 

[the snooper]

Pretty much the same as if you were holding the grinder.

or exactly the same as the cooker hood

 

[AndyStobbs]

As rtbcomp says. But you've already told me I can (literally) trust double insulation with my life Snooper.

 

[AndyStobbs]

You can't compare the two like that, metal conducts, plastic insulates.

 

[the snooper]

As rtbcomp says. But you've already told me I can (literally) trust double insulation with my life Snooper.

correct but my point is you would like to earth the hood but not the bench and in reality they are no different in that both are double insulated and can come into contact with steel but you would rather only earth on item

 

[the snooper]

You can't compare the two like that, metal conducts, plastic insulates.

exactly the grinder is plastic body as will be the fan assembly in the extractor hood

 

[AndyStobbs]

Aye but its full of wires, switches, a light etc. We will have to beg to differ.

 

[the snooper]

Aye but its full of wires, switches, a light etc. We will have to beg to differ.

ok

 

[brightspark]

if its double insulated DONT EARTH IT simple