current transformers

[gamblor6]

what are they and why fit them?
apparently scottish power insist on them if your over 100a per phase.
anyone know why?

 

[Marvo]

If you use direct metering the meters start getting large and cumbersome if they're over 100Amps.

 

[gamblor6]

how do they work tho marv?

 

[yellowvanman]

Its like a clamp meter (or is one) - a winding is placed around the conductor and the ac induces a current in the secondary (metering side) which will be a smaller more easily measured current.

 

[gamblor6]

so kinda one in 10 is counted?

 

[yellowvanman]

Something like that!

 

[gamblor6]

cheers guys

 

[Marvo]

They're a very accurately wound transformer that clamps or sits around the wire carrying the current. Depending on the ratio of the primary / secondary windings of the CT, if 100 amps is flowing in the current carrying wire then the CT might give a 5 Amp output on the secondary coil. The metering device measures this 5 Amp output and if the ratio is correctly set it will meter the current being supplied as 100Amps.

 

[Knobhead]

See Current transformer - Wikipedia, the free encyclopedia
Just a word of waning, never open circuit a CT with out isolating the supply first. They can give out very high voltages.