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Topic Title: Ct's
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Created On: 18 February 2013 12:35 PM
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 18 February 2013 12:35 PM
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Martynduerden

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I am changing a few tp boards, now one of the existing boards extension boxes has been used to house a set of CT's for metering to all the boards.

All the boards are fed from a 160A tap off on a rising main.

Can I move the Ct set in the tap off enclosure - there is masses of space.

My instinct says I cannot use the tap off as an enclosure for other equipment, however it is for the equipment supplied by the tap off.

The CT's and metering arrangements are private use only.

What is the panels opinion on reducing the CSA of single cores after the CT's to each board - there is no additional upstream or downstream protection against overload or short circuit

(With the obvious exception of 48 mcb's in each board)

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Martyn.

Only a mediocre person is always at their best



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 18 February 2013 06:30 PM
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Martynduerden

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Any thoughts fellas?

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Martyn.

Only a mediocre person is always at their best



www.electrical contractors uk.com
 18 February 2013 06:37 PM
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ArthurHall

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I would think you would be OK fitting the CT's in the tap off box, as for reducing the size of the conductors after the CT's, asuming the tap off box is fused keep the tails below three meters and double insulate or enclose in earthed metal.
 18 February 2013 06:52 PM
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Martynduerden

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Originally posted by: ArthurHall

I would think you would be OK fitting the CT's in the tap off box, as for reducing the size of the conductors after the CT's, asuming the tap off box is fused keep the tails below three meters and double insulate or enclose in earthed metal.


The tap off is fused at 160A, the "tails" are 95mm through the CT's then 35mm to each board.

Obviously 35mm is not good for 160A by my calcs.

I tend to find myself thinking "I must be wrong"!

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Martyn.

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 19 February 2013 09:31 AM
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OMS

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You can't be anything yet Martyn - you don't know the load, or if it has an overloading characteristic. 35mm2 is pretty resilient on 160A protection if you aren't delivering 160A into a system that doesn't demand 160A

The tap off fuses may only need to offer short circuit protection - what's the bus bar rating and what the feed in protection.

CT's in the tap off - no real drama - just be mindfull of leaving them open circuit (or them going open circuit. Personally, I'd want them iun an enclosure with a test/connection block with shorting links available

Regards

OMS

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 19 February 2013 09:32 AM
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AJJewsbury

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Obviously 35mm is not good for 160A by my calcs.

It could well be OK for fault-only protection - depending on the upstream protective device, Zs etc.

Would the "characteristics of the load" give overload protection? (433.3.1 (ii))

- Andy.
 19 February 2013 10:39 AM
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Jaymack

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Originally posted by: Martynduerden
I am changing a few tp boards, now one of the existing boards extension boxes has been used to house a set of CT's for metering to all the boards.


For private use, are they still being used for metering purposes? If not I would recommend scrapping them. If still required, I would be looking at the characteristics of the CT's - turns ratio, VA rating, class, accuracy on the curve, knee point and the secondary cable size etc., all in relation to the expected loadings for the new duty.

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 19 February 2013 09:02 PM
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Martynduerden

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Originally posted by: OMS

You can't be anything yet Martyn - you don't know the load, or if it has an overloading characteristic. 35mm2 is pretty resilient on 160A protection if you aren't delivering 160A into a system that doesn't demand 160A.


I do know the load, I clamped the tails at 81A power board and 36A lighting board, I what I don't know is If they are peak loadings.

Overloading is fairly easy, everyone plugs in heaters until the UPS sends out its overload message!

Now the lighting board loading is ok in relation to overload as this is unlikely to reach anything like 160A and if it was likely the whole install would be up of re-design in any case!

Short circuit, well I can only work from upstream of the tap off, so that is an unknown at the moment.

The tap off fuses may only need to offer short circuit protection - what's the bus bar rating and what the feed in protection.


1200A, I cannot gain access to the supply end, and under no circumstances could I isolate it to inspect the protection.

Must assume that the tap off affords both SC and OL protection.



CT's in the tap off - no real drama - just be mindfull of leaving them open circuit (or them going open circuit. Personally, I'd want them iun an enclosure with a test/connection block with shorting links available

Regards

OMS


Am I missing something, are they more likely to go open circuit in the tap off, obviously shorting blocks will be fitted!

I am we'll aware of the dangers of open circuit secondaries ... Unfortunately!

-------------------------
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Martyn.

Only a mediocre person is always at their best



www.electrical contractors uk.com
 19 February 2013 09:06 PM
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Martynduerden

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Originally posted by: Jaymack

Originally posted by: Martynduerden

I am changing a few tp boards, now one of the existing boards extension boxes has been used to house a set of CT's for metering to all the boards.


For private use, are they still being used for metering purposes? If not I would recommend scrapping them. If still required, I would be looking at the characteristics of the CT's - turns ratio, VA rating, class, accuracy on the curve, knee point and the secondary cable size etc., all in relation to the expected loadings for the new duty.

Regards


They are for metering and must be retained.

Why would I need to look at them in any detail?,essentially this is a distress change as the boards are physically damaged, the capacity of the system is not changing they have a 160A ASC, and will not be going beyond the original design loading.

-------------------------
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Martyn.

Only a mediocre person is always at their best



www.electrical contractors uk.com
 19 February 2013 09:11 PM
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Martynduerden

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Originally posted by: AJJewsbury

Obviously 35mm is not good for 160A by my calcs.


It could well be OK for fault-only protection - depending on the upstream protective device, Zs etc.


Would the "characteristics of the load" give overload protection? (433.3.1 (ii))

Whilst I can be fairly sure 433.3.1 (ii) could be applied for the lighting board, the same could not be said for the power board, do you think 35s fit for SC? -I haven't actually checked as I will be removing / altering them.


- Andy.


-------------------------
Regards

Martyn.

Only a mediocre person is always at their best



www.electrical contractors uk.com
 20 February 2013 08:25 AM
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Jaymack

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Originally posted by: Martynduerden
Why would I need to look at them in any detail?

If my COTB, I would be verifying that they are/were correct for the application, whether or not there is no load change.

Regards
 20 February 2013 10:24 AM
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OMS

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Originally posted by: Martynduerden

Originally posted by: OMS

You can't be anything yet Martyn - you don't know the load, or if it has an overloading characteristic. 35mm2 is pretty resilient on 160A protection if you aren't delivering 160A into a system that doesn't demand 160A.


I do know the load, I clamped the tails at 81A power board and 36A lighting board, I what I don't know is If they are peak loadings.

Are they credible loadings then Martyn - how do they relate to the floor area served or the number of workstations.


Overloading is fairly easy, everyone plugs in heaters until the UPS sends out its overload message!

So the power board is backed up by UPS ? after the tap off ?


Now the lighting board loading is ok in relation to overload as this is unlikely to reach anything like 160A and if it was likely the whole install would be up of re-design in any case!

OK - so that's one set of 35mm2 you can be pretty confident over then

Short circuit, well I can only work from upstream of the tap off, so that is an unknown at the moment.


Have you taken a L-E and L-N loop at the tap off ? - you could quickly deduce PSCC at that point and see if 35mm2 is protected by 160A fuses - it almost certainly will be

The tap off fuses may only need to offer short circuit protection - what's the bus bar rating and what the feed in protection.


1200A, I cannot gain access to the supply end, and under no circumstances could I isolate it to inspect the protection.

OK - it appears to be a pretty robust system so overlaoding probably isn't credible at that level.


Must assume that the tap off affords both SC and OL protection.

OK - if you are making that assumption then fine - can yo change the bus bar protection rather than the cables - ie down fuse to suit the load and cable.





CT's in the tap off - no real drama - just be mindfull of leaving them open circuit (or them going open circuit. Personally, I'd want them iun an enclosure with a test/connection block with shorting links available



Regards



OMS




Am I missing something, are they more likely to go open circuit in the tap off, obviously shorting blocks will be fitted!

No more likley Martyn - just a word of caution that's all.


I am we'll aware of the dangers of open circuit secondaries ... Unfortunately!

OK - I've seen plenty of systems where the installer hasn't been - sounds like you've found a few as well



Regards

OMS

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Failure is always an option
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