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

 

[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