Just in from Square D [RE-wrenches]

hugh piggott hugh.piggott at enterprise.net
Mon Mar 12 08:57:38 PST 2001


<x-flowed>At 7:11 am -0800 12/3/01, Drake Chamberlin - Electrical Energy wrote:
>At 01:00 AM 03/12/2001 -0800, you wrote:
>>At 8:27 am -0800 11/3/01, Drake Chamberlin - Electrical Energy wrote:
>>>
>>>Part of the reason is the in the US, our National Electrical Code 
>>>requires that all ungrounded conductors have overcurrent 
>>>protection. I don't know how this would apply to British systems.
>>
>>Yes it does, but that's not a reason for the requirement, it's just 
>>a requirement.  While it is useful to know the wording of the code, 
>>I am actually more interested in knowing the logic behind it.
>
>A grounded conductor is at ground potential.  Current is initiated 
>through the wire that is energized relative to ground.  With an 
>ungrounded circuit,  either wire could potentially become the motive 
>force.  I think we are saying essentially the same thing.

Sorry to drag this on, and I don't suppose we'll reach any blinding 
insights, but as far as I can understand it a wire can only be 
energised relative to ground if the other wire is grounded by a 
fault.  This begs the question 'Which side of the breakers is this 
first ground fault?'

If it is on the load side of the breakers then one single pole 
breaker will do to clear the fault.  If it is on the supply side of 
the breakers then we need 2 breakers: I conclude that we use breakers 
on both wires to guard against one of the supply terminals (upstream 
of the breaker) becoming accidentally grounded at source without 
being noticed.  So far I hear no other suggestions, so I guess that 
must be the case.  I need to understand stuff like this, I can't just 
go by the code.  So often with renewables the code does not 
anticipate the situation you find yourself in, and makes an ass of 
itself.
-- 
Hugh

Scoraig, Scotland
http://www.scoraigwind.co.uk

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