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Topic Title: 2 separate tncs supplies being used by one dwelling.
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Created On: 01 October 2013 08:04 PM
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 01 October 2013 08:04 PM
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hertzal123

Posts: 300
Joined: 26 August 2007

An outdoor centre where I have worked has a tx in a nearby field and 2 separate overheads feeding the adjacent meters.I am wondering if there is a danger of current from 1 supply flowing back thro,or sharing the neutral of the other supply and the safety implications of this.The supplies
were required for a large cooking and e7 load.The supplier stated there
is something adrift with the meter readings and I am wondering if a neutral imbalance could be the cause.
Thanx for any advice.
Hz.
 02 October 2013 09:11 AM
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broadgage

Posts: 1283
Joined: 07 August 2007

Two supplies to the same premises is often "frowned on" but happens suprisingly often in extensive buildings, or those that were previously two or more homes or busineses.

Care should be taken to keep the two supplies electricly seperate, and possibly to provide warning notices advising that the premises has two supplies.

Interconnection between the two supplies could in theory overload the neutral of one supply, but is seldom a problem unless very substantial load is connected to the wrong neutral.

I doubt that incorrect meter readings would result.
Each meter measures the current in the phase conductor and the voltage between phase and neutral. The meter does not "know" what happens to the current after it passes through the meter and the load.
The current could return via the correct neutral, via the other neutral, via the CPC or even via the general mass of earth, the meter will still correctly record it.

Suspect meter readings are more likely to result from meter reading or admin c*ck ups, including
Reading meter "A" and applying the reading to calculate the bill due on meter "B"
Getting the "day" and "night" readings the wrong way round
Charging all the usage at a flate rate when it should be at different rates for day or night.

Or of course "suspect" meter readings may be correct if useage has greatly increased or declined, or if numerous previous bills have been estimated but a reading has now been taken.
 02 October 2013 09:25 AM
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AJJewsbury

Posts: 11379
Joined: 13 August 2003

If the two supply CNEs are from the same source and are of the same length, then they should share the current OK - even if they're different c.s.a.s. (the current share should be in proportion to the conductivity - i.e. pretty much the ccc). It could get nasty if one CNE goes open circuit though - but that's probably more the supplier's lookout rather than yours. Just make sure the bonding is chunky enough.
- Andy.
 02 October 2013 03:07 PM
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Avatar for Parsley.
Parsley

Posts: 993
Joined: 04 November 2004

This might be worth a read.

http://library.ukpowernetworks...n+Earthing+Design.pdf

See section 7.8.4 and 7.8.5

Regards
Statistics

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