Current transformer metering producing implausible readings

BrianHJR · Jan 15, 2022

Petej999 said:
Are the CTs of the correct ratio and compatible with the meters???

We don’t know. We aren’t going to mess with the meters because that would open us up to accusations of tampering…
I was posting on here to get some ideas of what might be wrong so that we can query this with the site management, who are otherwise pretty clueless.

Julie. · Jan 15, 2022

BrianHJR said:
We isolate the main switch on the incomer to the building.
In what way

Review the attached.

There is no guarantee that the switch operates and isolates correctly, therefore you must check before working on anything

nicebutdim · Jan 15, 2022

BrianHJR said:
Of course. We have a specific iso padlock for doing just that.
Moderately pointless though, because the main isolator is right underneath the bus bar and so anyone turning it back on would have had to fight their way past us first…

I'd use similar logic with regards to locking off when I'm the only person in a building but, unlike cats, we only have one life and I'd prefer to go beyond reliance on a switch when ensuring isolation of supply.

timbobelfast · Jan 15, 2022

BrianHJR said:
Edited - please read right to the end!

Grateful for any advice regarding CT metering which seems to be generating inaccurate/implausible readings.

Our charity has a commercial building with 3-phase connection into a busbar chamber at one end - let’s call us ABC.
The rest of the chamber supplies three other switched fuses supplying another business nearby with 3 x single phase supplies - let’s call it XYZ.

The landlord had a contractor install 2 x Socomec Countis E42 units to meter our usage separately from the other business. They were set up so that XYZ meter records total usage, and ABC meter records only our usage.

Our ABC meter did not register any usage from day one, we let them know but they did nothing for months. Eventually they got the same contractors out to investigate and repair.
Since they did this our meter works, but the readings seem implausibly high, especially when compared to the XYZ usage.

Recently one of the directors observed that in 2.5 hours, our ABC meter incremented by 3KWH with just a few fluorescent light tubes on.
In the same time period, the business XYZ only had 1 KWH increment on their meter, despite being observed to have far more potential usage.

Does anyone have any experience of current transformers being inaccurate?

Could the issue be that we are using all 3 phases?

Might it just be a really bad or problematic install?

The landlord has tried to tell us it’s fine, but they did eventually say they’d get the contractors back out to have a look… we have considered installing our own check-meter but it’s a cost we really don’t need!

I’ve attached images of the install before the repair - I’ve no reason to believe they’ve changed the position of the CT clamps but I’ve not had a chance to look inside the bus bar chamber recently…

Our ABC connection is on the far right - the other 3 connections run to XYZ supplies…

Thanks in advance.

EDIT: Having thought about it a bit deeper, I’m pretty certain that they must have adjusted the positions of the CTs as the metering is now supposed to be completely separate… so the image will not be terribly useful in diagnosing the latest issues.
I’ll try to get an updated photo when I can… unfortunately it’s a two person job to remove the BBC cover since they mounted the meter box onto the front of it!

View attachment 94006 View attachment 94006

As already mentioned, check the CT ratio is correct. Verify they have not used protection type CTs by mistake as these are normally not suitable for metering!
And those secondary connections on the CTs need earthed as well for safety!

pc1966 · Jan 16, 2022

Provided there is nothing else to the right of your fused-switch then the CT location is correct, even if putting them on the busbars a bit off. However as already pointed you the meter has to be set with the correct ratio so you get charged for 1A * 230V and not 5A * 230V if the ratio is out by 5, etc.

The CT for the main supply looks odd, two of them seem to be on the incoming supply taps but 'blue' L3 is off the busbar as nothing to the left seems to use it.

Final question, how are the metering cables taking out with the cover on? Please tell me they are not crushed under the lid?!!! There is no adequate protective fusing of the voltage tap meter cables so they should be subject to additional protection against mechanical damage / shorting out / etc!

Petej999 · Jan 16, 2022

Julie. said:
Review the attached.

There is no guarantee that the switch operates and isolates correctly, therefore you must check before working on anything

ccan't open it

Julie. · Jan 16, 2022

Petej999 said:
ccan't open it

Oops

It's the best practice guide 2 from electrical safety first.

https://www.----------------------------/professional-resources/best-practice-guides/

Could no-one else open it? Works for me

pc1966 · Jan 16, 2022

Julie. said:
Oops

It's the best practice guide 2 from electrical safety first.

https://www.----------------------------/professional-resources/best-practice-guides/

Could no-one else open it? Works for me

Works OK for me as well.

Using Firefox on desktop so maybe not so good on mobile/tablet?

BrianHJR · Jan 16, 2022

Julie. said:
Review the attached.

There is no guarantee that the switch operates and isolates correctly, therefore you must check before working on anyth

pc1966 said:
Provided there is nothing else to the right of your fused-switch then the CT location is correct, even if putting them on the busbars a bit off. However as already pointed you the meter has to be set with the correct ratio so you get charged for 1A * 230V and not 5A * 230V if the ratio is out by 5, etc.

I’d certainly not trust the meter installers to have got it right. Neither of our NICEIC people thought very much of the install when they visited.

pc1966 said:
The CT for the main supply looks odd, two of them seem to be on the incoming supply taps but 'blue' L3 is off the busbar as nothing to the left seems to use it.

Yes that strikes odd, they have done it like that because there wasn’t enough space to clamp the grey phase incomer.
And you’re correct, XYZ only run on the brown and black phases.

pc1966 said:
Final question, how are the metering cables taking out with the cover on? Please tell me they are not crushed under the lid?!!! There is no adequate protective fusing of the voltage tap meter cables so they should be subject to additional protection against mechanical damage / shorting out / etc

The cables go through the front of the chamber and into the back of the meter box through a brass bush fortunately.

Very good point about the metering wires, they should presumably be insulated and sheathed like the rest of the cabling within the chamber?

I’ve attached a couple of photos of the inside of the meter box showing the wires supplying the power for the meters via an MCB - you can hopefully see that they have several cuts on them, fully down to the conductors - as left by the installers.
also a minor point but should the metal backplate in the meter box be bonded?
There‘s currently no proper bonding to the front cover of the bus bar chamber either, other than that tenuously provided by the fixing bolts…

[ElectriciansForums.net] Current transformer metering producing implausible readings

BrianHJR · Feb 20, 2022

So the electrical company were sent back to investigate the metering issues and fitted completely new ones in a different location, rather than bolting the meter box to the front of the bus bar chamber lid they have put it above on the wall.
The install seems to be of a significantly higher standard this time, but there is one potential glaring issue if we ever decide to use the spare switch fuse for anything else in future…

They didn’t leave any instructions for getting the meter readings and I had to work that out myself today.
A much better idea would have been for them to mount the metering box outside on the external wall, so that they wouldn’t have to rely on us for the figures and could send someone out to periodically check it?

The good news is that the meters are now producing plausible readings. The instantaneous import reading for our building showed a KW figure which was completely reasonable for the demand at that point and so we can hopefully now assume that the CTs are set up correctly…