From: peter2@hipson.net
On Tue, 09 Mar 2010 14:02:20 +0000, Tim Watts wrote:
>PeterD
> wibbled on Tuesday 09 March 2010 13:14
>
>> On Tue, 09 Mar 2010 06:55:17 +0000, Tim Watts wrote:
>>
>>>Jon
>>> wibbled on Tuesday 09 March 2010 06:41
>>>
>>>> All I need is a dpdt knife switch. If I was cheap I'd get some copper
>>>> and make one myself.
>>>>
>>>> I just bought an economical manual dpdt transfer switch rated for 100a
>>>> at
>>>> 240vac. I might have to have the electric company disconnect service so
>>>> I can wire it in.
>>>>
>>>> Don't tell me now I did something wrong. What?
>>>>
>>>
>>>Don't you need 3p (two lives and a neutral)?
>>
>> Never switch neutral.
>>
>>>
>>>Have you found out how you need to handle the ground/earth connection?
>>>Here, in the event of a power failure,
>>
>> See above...
>>
>>>the utility earth cannot be trusted,
>>
>> You're required to have ground on your premises at the entrypoint.
>> Typically three ground rods.
>>
>>>so for
>>>a generator hookup, one needs a local earth rod or rods bonded to the
>>>neutral of the generator.
>>
>> Not just generator, but all service. And that's why you never switch
>> neutral.
>>
>>>As the earth loop fault impedance is now too high
>>>(typically 100+ Ohms for earth rod impedance) to trip a type B or C
>>>breaker in a timely manner, an RCD (you call them GFCI I think) must also
>>>be employed). What does your code say?
>>
>> (Comments based on USA rules, may vary in other countries!)
>
>That is interesting. Under the IEE Regs (UK), the neutral is considered a
>live (ie dangerous). To distinguish what most people think of as a "live",
>the other supply conductor is termed "phase", or more recently "line".
Hence my disclaimer! In the US, the neutral is connected to ground
in the power system in several places, including at the pole, and
again at the entry/box. We're very big on keeping it at ground
potential. I'm sure the UK system is different, as you describe.
>
>Thus, we are required to switch out the neutral in the OP's scenario, which
>is pertinent, as with certain types of supply fault on our 3 phase system,
>the neutral can potentially move up to 240V away from earth (and lead to a
>local 1 phase outlet experiencing a large voltage between the live and
>neutral, eg >300V - a scenario I have witnessed).
>
>We have an interesting edge case due to having three different types of
>utility earth provision:
>
>1) None - (usually overhead wires, pole transformer nearby) - customer
>provides their own earth via rods or similar;
>
>2) TN-S where the earth conductor is continuous all the way back to the last
>transformer (which is typically an 11kV-240V 3 phase jobbie supplying
>several roads). This is a pretty bomb proof system for underground cabling
>but is less popular as it requires more metal in the cables. Originally the
>lead serving (sheathing) on the old paper/tallow cables maintained the
>earth, but even where such cables still exist, the lead is rotting away and
>becoming unreliable.
>
>3) TN-C-S - earth and neutral are the same conductor until the customer
>premsises where they are separated.
>
>Case 3 is the fiddly one - although the earth/neutral is rodded down to real
>earth periodically by the supplier, it is not fully reliable in the case of
>certain faults and could be pulled away from the local earth potential by
>the wrong type of supply side fault. I'd have to read the generator part of
>the regs to see what to do with this case here, but it's not clear cut for
>reasons given.
>
>The motto is of course, nothing should be taken as obvious until the
>regs/codes have been read :)
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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