Meter Swap over - CT Question

[Mrbilbo25]

Morning all,

Really hoping to get some advice / confirm what a few of us are thinking about a job in the future.

Back ground - We have to swap over existing 3 phase meters on DBs throughout a building, however due to the nature of the clients work we are unable to kill the supply to each board and work dead.

Our main concern is those pesky CTs that are currently connected to the old meters, these will be staying and reused on the new meters, however to get the new meters to fit we need to do some work etc so the CTs will be disconnected from any meter for around an hour potentially which we know poses a serious issue.

In an ideal world i know we would turn off the supply and work dead but as i said that is not an option, if we were to remove the leads that go to each CT from S1 and S2 on the old meter, link them together to short them out and then also take a link down to earth would this be the best solution or is there a better way to proceed?

Any advice would be greatly appreciated.

Cheers

 

[timhoward]

Welcome to the forum.
I’m going to be a bit direct - in the scheme of things my biggest concern wouldn’t be the CT clamps.

Have you thought through your plan considering regulation 14 of EAWR?
It’s not just ‘in an ideal world’, in almost all cases you are legally obliged to isolate.
If something goes wrong it feels as though this plan would be quickly deemed illegal and any consequences of an accident would rest with whoever is telling others to work live on a 3 phase supply.
(The client ceasing to make money does not count as ‘unreasonable’!)

Sorry to be blunt, and I know this wasn’t what you asked, but as it might save a life so I’ll risk annoying you!

 

[westward10]

They shouldn't pose a threat when disconnected but if you are worried put connectors on each end. I wouldn't short them out.

 

[Mikegh]

Could be a medical facility or similar

Whatever there should be a plan for these works.

 

[Mrbilbo25]

Welcome to the forum.
I’m going to be a bit direct - in the scheme of things my biggest concern wouldn’t be the CT clamps.

Have you thought through your plan considering regulation 14 of EAWR?
It’s not just ‘in an ideal world’, in almost all cases you are legally obliged to isolate.
If something goes wrong it feels as though this plan would be quickly deemed illegal and any consequences of an accident would rest with whoever is telling others to work live on a 3 phase supply.
(The client ceasing to make money does not count as ‘unreasonable’!)

Sorry to be blunt, and I know this wasn’t what you asked, but as it might save a life so I’ll risk annoying you!

The client ceasing to make money is not an issue, these boards feed other equipment that can't be shut down so they would meet the necessary requirements for working live!

Sorry to be blunt but you've assumed it was a financial issue and without knowing full details, all necessary precautions that can take place will be taking place. The 3 phase supply to the meters can be isolated etc it's the CTs that are causing the issue which we have found a workable solution too.

 

[timhoward]

The client ceasing to make money is not an issue, these boards feed other equipment that can't be shut down so they would meet the necessary requirements for working live!

Sorry to be blunt but you've assumed it was a financial issue and without knowing full details, all necessary precautions that can take place will be taking place

That is entirely fair and I apologise.
I realised what you were actually asking about 20 minutes later, after I jumped the gun.
Hope it goes well!

 

[Mikegh]

Bit confusing. I thought you said you couldn't kill the supply

 

[UNG]

They shouldn't pose a threat when disconnected but if you are worried put connectors on each end. I wouldn't short them out.

Shorting CT's is very basic electrical knowledge to prevent electrical runaway
They must be shorted before they are disconnected from the meter or the device they are connected to otherwise they are likely to be damaged while they are O/C and not reuseable. There is also the risk of a severe electric shock from an O/C CT

 

[Mrbilbo25]

Shorting CT's is very basic electrical knowledge to prevent electrical runaway
They must be shorted before they are disconnected from the meter or the device they are connected to otherwise they are likely to be damaged while they are O/C and not reuseable. There is also the risk of a severe electric shock from an O/C CT

Glad to see this is such a friendly forum and not just a place for people to belittle and shoe off. Jesus christ, think I'll go else where to confirm our plan. 40 years in the industry and still have to deal with know it alls

 

[timhoward]

Shorting CT's is very basic electrical knowledge to prevent electrical runaway
They must be shorted before they are disconnected from the meter or the device they are connected to otherwise they are likely to be damaged while they are O/C and not reuseable. There is also the risk of a severe electric shock from an O/C CT

Help me out here. Is a ct clamp not just a load of thin wire with both ends connected to something that measures the induced current?
I get they could hold a capacitive charge but how can they be damaged bu failing to short them?

 

[davesparks]

Help me out here. Is a ct clamp not just a load of thin wire with both ends connected to something that measures the induced current?
I get they could hold a capacitive charge but how can they be damaged bu failing to short them?

When they are installed around a conductor that conductor forms the primary winding of a transformer. So you have a transformer with a single winding primary and many many winnings secondary, the result being that if you disconnect the secondary whilst the primary is live the open circuit voltage of the secondary will be many times that of the primary.

Disconnecting without shorting the secondary leaves you with a few thousand volts between the terminals which is what causes the danger and potential damage.

 

[Marvo]

I would suggest you never open circuit the secondary circuit of a current transformer. If you do the voltage across the secondary coil can rise extremely high if there a load on the circuit it's clamped around. Sometimes CT's have built-in voltage clamping protection against this but don't ever assume that's the case. The high voltage that developes can damage the CT and it can also damage the person that's in the vicinity.

 

[davesparks]

Glad to see this is such a friendly forum and not just a place for people to belittle and shoe off. Jesus christ, think I'll go else where to confirm our plan. 40 years in the industry and still have to deal with know it alls

He was replying to westwards poor advice above, not your original question.

Nobody is trying to belittle you or show off.

The issue with your plan at the moment is that you need to have the short in place before disconnecting the existing meter.
The problem is that the moment you disconnect the CT secondary it will rise up to many thousands of volts.

 

[westward10]

Yes I was wrong didn't think of how high it could rise.

 

[UNG]

Glad to see this is such a friendly forum and not just a place for people to belittle and shoe off. Jesus christ, think I'll go else where to confirm our plan. 40 years in the industry and still have to deal with know it alls

What point are you trying to make
How to deal with CT's was something that I was taught in my first year at college back in the 70's my comment was not to belittle but to stress the dangers of handling CT's when they are live of which @davesparks has pointed out the pitfalls

You might find that the know it alls as you call them will be here tomorrow when the others that blindly do a job don't due to the unknown issues that bite them

With 40 years in the industry you will still find there are people out there that know more than you rather than being offended my advice would be learn from it