Removing Split Core CT's

[Mark.W]

Hi peeps,

I need to remove some split core CT's from a school's Main Tails. The installers never used a shorting block but did earth the secondaries in the meter. I have tried unclipping the CT's but the magnets aren't releasing.

Can one of you kind people please advise me as to how I can get them off. Turning the supply off is not an option at this time.

I must add that this is an unofficial export meter that was installed a solar panel company.

Regards
Mark

 

[shanky887614]

ct's, shorting block?

 

[Knobhead]

One leg of the CT’s should be earthed to form a common link. To handle them safely they should be shorted at the O/P terminals, preferably to earth.
What was the loading when you tried to remove the CT’s? I’ve only ever removed them under no load. Can you not leave them shorted until such a time you can kill the supply? By shorting at the CT's at least you can remove the meter.

Just to cheer you up

 

[Lucien Nunes]

I doubt it's magnetism holding them together, if you really can't separate the cores. No sneaky hidden fasteners?

ct's, shorting block?

Special terminal block to allow disconnection of the meter or instrument while there's still load on the CTs, by putting screws / links in to short the secondaries without any risk of open-circuiting them in the process.

A method I used in emergency was to cut away an inch of insulation from each of the secy leads and clamp them together in slotted line taps such as Legrand http://docdif.fr.grpleg.com/general/legrand-exp/CEXP2012-13/ex212001_617.pdf

 

[Mark.W]

The loading was about 200A per phase! Like I said there was no shorting block and not so sure about shorting them now that they have current flow through them!

 

[Mark.W]

Hi Lucien, there's definately no hidden clips!

 

[Mark.W]

The problem is these CT's are buzzing and getting quite warm, I need to remove them and really can't turn the supply to the school off!

 

[Bobster]

I've never had a problem removing split CTs even with a couple of KA being monitored.

 

[Lucien Nunes]

buzzing and getting quite warm

Is the meter still present / functioning? If not, are you sure the secondaries are shorted?

 

[Mark.W]

The meter is still present and working! S2 from each CT is earthed in the meter terminals!

 

[Marvo]

If those secondaries aren't shorted then there's a good chance they could start a fire and there could be some very high voltages present. If they're buzzing and getting warm/hot then I'd take that as a sign they might not be shorted. I get nervous with these CT threads we get because CT's are a lot more dangerous than many people give them credit for.

Do a risk assessment and when you've weighed up the hazards then make a plan with the school for an early morning power outage in order to remove them safely. If you're determined to go ahead as a live job then it should be two persons present and full ppe.

 

[Mark.W]

Also noticed that the CT's are 100:5 where SSE CT's are 300:5

 

[Lucien Nunes]

If those secondaries aren't shorted then there's a good chance they could start a fire and there could be some very high voltages present

The meter is connected and working so they are shorted by the meter at the moment. But it sounds like they are undersize for the job, hence the heating.

 

[Knobhead]

As Marvo said, arrange a time when the supply can be disconnected no mater how inconvenient it is. I’ve had underrated CT’s catch fire, if that were to happen the supply will be lost anyway.

It’s up to you but I wouldn’t hesitate to kill the supply.

 

[Stu1312]

If they are being overloaded I suggest a pair of LV gloves and get them off quick if isolation is difficult.

 

[UKMeterman]

Some people are forgetting what a transformer means, these are current transformers and the seondary behaves like a current source, open cicruit it under load and you will get a very high voltage on the seondary, enough in some cases to cause the insulation to breakdown.

You have no choice you need to get them off supply for an hour or so. Perhaps the overloaded magnetic circuit is pulling them hard shut....

 

[Darkwood]

Firstly if expressed that the potential for a fire hazard is high due to the dangers and possible incorrect sizing of the CT's then I'm sure you will get permission for a power down.... Ive never been to any school where powering down wasn't something that could be arranged.

Don't let the cart lead the horse here, your the professional.

 

[Mark.W]

Ok so I am going to switch off the supply possibly this evening, how long do I need to wait before attempting to split the CT's?

Thanks for everyone's advice, been great as usual.

Mark

 

[Bobster]

If you've got the secondary shorted, there's nothing different to you using a clamp meter.

 

[Mark.W]

If you've got the secondary shorted, there's nothing different to you using a clamp meter.

S2 from each CT is linked at the meter and run to earth, so they are shorted at the meter. So what you are saying is as soon as I power down I can split them?

 

[Bobster]

To short the secondary both S1 and S2 need to be linked together preferably to earth.

At the moment they are linked within the meter they are wired to.

 

[Mark.W]

Thanks RoB2,

I have also switched the meter off for the time being but left the CT's connected, is this ok?

 

[Bobster]

I haven't seen the installation myself so I can't guarantee anything over the forum, but yes. Even when powered down the meter will still keep the secondary of the CT as a closed loop.

It's only when the secondary goes open circuit that high voltages can become present.

If the CT has been overheating badly as you have suggested, it could well be possible the plastic surrounding the CT could have warped, possibly causing them to be difficult to remove.

 

[Mark.W]

Thanks mate,

I had a look at them yesterday, they seem in tact and warm to the touch but not scoulding hot. I will power down this evening and remove them. Just to clarify once powered down I should just be able to unclip them immediately?

Thanks for your help, much appreciated.

 

[Bobster]

Yes once powered down, just unclip them.

 

[Marvo]

......If the CT has been overheating badly as you have suggested, it could well be possible the plastic surrounding the CT could have warped, possibly causing them to be difficult to remove.

This was also my thinking, unless there's a hidden fixing I couldn't see any other reason they'd be difficult even when under load.

I'm glad you arranged to power down Mark, it's far safer. Did you managed to get them off without a hitch?

 

[Mark.W]

Hey Marvo,

The school is only allowing me to power down on Friday evening now! We are however keeping an eye on the CT's.

Cheers
Mark

 

[Mark.W]

Thanks for all the support guys.

The school has now asked that I put correct size CT's on this meter and looking at the current setup it looks as if the main incomers are fused as follows:
Red - 300A
Yellow - 500A
Blue - 300A

Now my question is do I size them all at 500:5 or can I use different ratio's ie. Red - 300:5 Yellow 500:5 and Blue 300:5?

Thanks again.

 

[Bobster]

Depends on what the power meter can be set up to, some can accept different ratios, others can't.

Personally as this isn't a high accuracy situation I'd fit 3 500:5.

Did you find why they were difficult to remove?

 

[Mark.W]

Hi RoB2

I have no idea, as soon as I powered down I was able to split them with no hassle. I gave them a good once over and there is no warping ect. It was mentioned in an earlier post about an overload magnetic circuit.

These are what were on there they are made by HOBUT

Thanks for your help, I was thinking down the same lines of 500:5 and as you say it's not a high accuracy situation, so 500:5 it is.

Cheers
Mark

 

[Mark.W]

Guys I am trying to find a supplier for shorting blocks but cannot find any, has anyone got a contact? Also a wiring diagram would be much appreciated if possible please?

Regards
Mark

 

[shanky887614]

Guys I am trying to find a supplier for shorting blocks but cannot find any, has anyone got a contact? Also a wiring diagram would be much appreciated if possible please?

Regards
Mark

have a google for

split core ct's uk


there are loads, if you want to actually pick them up ask a wholsaler if they can order some in.

online would be your best bet

 

[Mark.W]

Thanks Shanky, but I am actually looking for the shorting block not the CT's.

Regards
Mark

 

[Stu1312]

All you need is a row of terminals that you can put a shorting strip across, this lot sell it complete: CT Operated Meter Installation Block - Test Terminal Blocks - Installation Equipment