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Topic Title: CT's
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Created On: 17 November 2012 12:43 AM
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 17 November 2012 12:43 AM
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calumbtw

Posts: 30
Joined: 22 October 2012

Hi,

Firstly, I'd like to say thanks to everyone that helped me in my last topic (didn't want to bump the thread). Not sure if this is the correct section to post this in either.

I'm a bit stuck regarding metering. On an assignment about a project with a 100A TP&N supply I've been asked to work out if the metering is whole current or CT.

I've done some research and I believe CT metering would be used, but I'm not quite sure how to work out the CT ratio.

I realise I probably haven't conveyed myself clearly, but any help is appreciated!

Thanks
 17 November 2012 01:17 AM
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Legh

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Joined: 17 December 2004

I've not seen CT's used on a fixed 100A or 200A 3-phase service heads before. (although i may be wrong). However, If the current supply is ramped up, and usually applies to exclusive transformers to individual installations then I would expect to see them.

Legh

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 17 November 2012 01:38 AM
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calumbtw

Posts: 30
Joined: 22 October 2012

Thanks for responding.

Is whole current usually used on 100A + 200A service heads? Please, would you be able to clarify why this is? Do you think there is any reason why CT's cannot be used on a 100A supply?

Thanks.
 17 November 2012 01:47 AM
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kj scott

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Joined: 02 April 2006

Whole current metering is on supplies up to 100A, after that CT coils are used. Never seen anything metered directly above 100A.

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 17 November 2012 07:48 PM
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ArthurHall

Posts: 737
Joined: 25 July 2008

You could use either but in my experiance the DNO's use total current up to 100A ratio metering after that.
To calculate the CT ratio is easy the primary should be similar to the full load so 100 or 150A the secondarys are normaly standardised on 1A or 5A, 5A being more common of this size. the meter would also have to be a 5A but scaled so that 5A flowing reads as 100A
 17 November 2012 08:12 PM
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UKPN

Posts: 548
Joined: 17 January 2012

--------we still see some of the old high load "whole current" installations
around, mainly over 100amp heating loads, school kitchens which required a 125 amp meter etc. they are fused at 160amp and cabled at
50mm.

arthur is correct though, there are a few 100amp ct operated jobs, probably where the load had been lowered and a "special" ct meter
of 100amp had been installed.

Regards.
 18 November 2012 03:39 PM
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slittle

Posts: 3597
Joined: 22 November 2007

Originally posted by: UKPN

--------we still see some of the old high load "whole current" installations

around, mainly over 100amp heating loads, school kitchens which required a 125 amp meter etc. they are fused at 160amp and cabled at

50mm.



arthur is correct though, there are a few 100amp ct operated jobs, probably where the load had been lowered and a "special" ct meter

of 100amp had been installed.



Regards.




There's a few of them on farms too, gradually they are being changed to CT which is always a bonus as we get to pick up the additonal work required on the customers side.

In answer to the OP, bear in mind depending how much detail you need to go into, that with CT metering you also need metering supplies to provide the V reference to allow the meter to calculate watts. This is normally achieved with small fused supplies taken from the cutout with the fuse holders mounted on the front of the CT chamber.

Stu
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