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Old January 27th 05, 12:34 AM
pompeysprk
 
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Default Electrical installation in my garden workshop

My Garden workshop is wired , 2 x 20a rings, 1 x 6a light, protected by
48amp rated rcd. the rings supply a 2kw rad and a computer desk with
periferals. The power is fed from a 32a mcb on my main house
distribution board .
Recently i lost power in the workshop, however there was no tripping of
any protective devices. I subsequently tripped and reset the mcb and
the rcd in the workshop and the power was restored . Intriqued with
this i isolated all the conductors from the mcb(house) and carried out
some insulation resistance checks and open/closed mcb operation checks
which were all fine .
However when i connect the main connector back to the mcb but leave it
switched off and carry out the same checks the open circuit
characteristic no longer exists when the 20 ring mcb are switched off.
Closed the resistance is .2 ohms but when opened the resistance is 56
ohms where i had previously had open circuit , ?????????????????? The
earth for the workshop is taken from the main house distrubution earth
..
I have reconnected everything in the workshop and have power available
with heavy load with no problems.
Whats going on.


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Old January 27th 05, 10:43 AM
Christian McArdle
 
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However when i connect the main connector back to the mcb but leave it
switched off and carry out the same checks the open circuit
characteristic no longer exists when the 20 ring mcb are switched off.
Closed the resistance is .2 ohms but when opened the resistance is 56
ohms where i had previously had open circuit , ?????????????????? The
earth for the workshop is taken from the main house distrubution earth


Insulation resistance checks must be done with the circuit to be tested
isolated from any circuits that may have devices attached. An off MCB is NOT
isolation. This means that all appliances, hi fis, RCDs etc. must be removed
before the test.

With the MCB off, it doesn't disconnect the neutral. There will be loads of
appliances in the house (presumably still plugged in?) that will provide
various connections between neutral and earth. They're current being fried
with 500V DC between neutral and earth every time you attempt a test. That
assuming you even isolated the house from the mains supply. If you didn't
there will be a connection between neutral and earth through your TT
earthing, or the supplier's TN earthing.

Christian.


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Old January 27th 05, 07:38 PM
pompeysprk
 
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Thanks for the info,
I kinda new that the neutral was still connected but included the info
in order to give a little background to the original problem. The power
in the w/shop is still ok now, but i worries me that none of the
protective devices were switched off / tripped and there was no power
available at the time of the fault. Anyone any ideas.



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