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Topic Title: Theory explanation pls
Topic Summary: 87v live to earth to an entire village
Created On: 06 December 2013 09:44 PM
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 06 December 2013 09:44 PM
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ady1

Posts: 766
Joined: 19 April 2005

Due to the floods etc
One of my adjacent villages with about 1000 houses was left with 87volts supply for over 12 hours.
When a customer said 'The idiots, they should have turned it off altogether - don't they know ohms law' ....
I couldn't really coment....
I presume the current will increase if the voltage is reduced - but can someone explain ?
Regards
Ady

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Resistance is futile.
 06 December 2013 10:59 PM
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HarryJMacdonald

Posts: 254
Joined: 15 May 2002

If the load is resistive the current will decrease and the power consumed will decrease by the square. W = V²/R.

Danger is to things like electric motors which may fail to start and burn out or run so slow that the cooling fans don't produce enough cooling.

Some things, like laptop computer power supplies, can cope with this sort of voltage variation but not many.
 07 December 2013 05:05 AM
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gkenyon

Posts: 4478
Joined: 06 May 2002

Originally posted by: HarryJMacdonald

Some things, like laptop computer power supplies, can cope with this sort of voltage variation but not many.
And of course, switched-mode PSUs will draw more current as they continue to deliver the same power - and that's a long way from Ohm's Law.

I think I'd unplug most of my stuff until the voltage level was restored - most domestic appliances won't cope with the reduced voltage for an extended period. Maybe, as Harry said, the PC, laptop supply and mobile charger could, but that's about it (even the PC monitor may not manage it as they may, depending on age, be produced and marketed along with TVs for a specific region).

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Eur Ing Graham Kenyon CEng MIET TechIOSH
 07 December 2013 01:57 PM
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alancapon

Posts: 5772
Joined: 27 December 2005

I can't think of a scenario for this. An 87V supply to a third of the village, 150V to a further third and 240 to the rest would be plausible. HV overhead supplies usually use three single phase fuses. If one fails, on the 11/0.4kV stepdown transformer you are left with one phase working properly, and the remaining two effectively in series giving roughly half voltage, depending on how the load is shared.

Regards,

Alan.
 07 December 2013 05:05 PM
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ady1

Posts: 766
Joined: 19 April 2005

Thanks guys
Alan - he said all the houses around him were at 87v, I would have expected 1 in 3 to be at fault ?
Still a bit confused

Regards
Ady

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 07 December 2013 05:16 PM
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alancapon

Posts: 5772
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I would expect two in three, but closer to half mains. If the supply is two phases of HV rather than three, you could conceivably achieve it with a wire on the ground. I would like to think that the DNO concerned had "sensitive earth fault" protection that would trip for a downed HV conductor.

Regards,

Alan.
 07 December 2013 06:55 PM
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Banners

Posts: 117
Joined: 06 October 2013

Sounds like one of those green initiatives to me.
 07 December 2013 06:58 PM
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slittle

Posts: 3523
Joined: 22 November 2007

Originally posted by: alancapon

I would expect two in three, but closer to half mains. If the supply is two phases of HV rather than three, you could conceivably achieve it with a wire on the ground. I would like to think that the DNO concerned had "sensitive earth fault" protection that would trip for a downed HV conductor.



Regards,



Alan.



I had "one" of two on the ground over the summer that the protection didn't see. It had been on the ground for at least half an hour before the farm in question decided to ask me what to do about it... They've been educated since !

We were only about 10 spans from the primary on a 2 phase tee from the 3 phase ring so I would of expected something to see it


Stu
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