Installing a CT on 3 core cable.

[Chrisbee]

My friend has just had a Growatt battery system added to his existing FIT PV system.

The installers couldn't get the WiFi dongle to connect to his WiFi so he asked if I'd try (I failed).

While there I noticed that the two CTs had each been clamped right around 3 core cables. See pics attached.
Also one was on the AC connection of the battery inverter and I'm sure that one should be on the house incoming tails.

We called the installers who are coming back on Monday.

My question is, how should we expect the CTs to be attached to just one core in a run of T&E or 3 core flex? I'm thinking some sort of break out box, maybe with terminations (wago?) inside, along with space to attach the CT around just the live core.

 

[binky]

CT clamps work by monitoring the magnetic electrical field around the cable and can determine direction of energy flow and strength. If you have live and neutral in the clamp, then you get enrgy flowing both ways through the clamp in equal measure, so it just cancels the magnetic field out and you get nothing. Makes you wonder about the competence of the installers?

 

[Chrisbee]

CT clamps work by monitoring the magnetic electrical field around the cable and can determine direction of energy flow and strength. If you have live and neutral in the clamp, then you get enrgy flowing both ways through the clamp in equal measure, so it just cancels the magnetic field out and you get nothing. Makes you wonder about the competence of the installers?

Indeed it does. I got the impression that it's not a local company but they've used a local subby.
Talking to what was probably head office, they very quickly agreed it was wrong.

So how would you expect the 3 cores to be broken into? If it's the same guy, I'm thinking we may have to make suggestions.

 

[binky]

Indeed it does. I got the impression that it's not a local company but they've used a local subby.
Talking to what was probably head office, they very quickly agreed it was wrong.

So how would you expect the 3 cores to be broken into? If it's the same guy, I'm thinking we may have to make suggestions.

ct clamps are normally fitted on the meter tails, have a trawl of the posts on here, there's plenty of example photos.

 

[Chrisbee]

Many thanks @binky

I guess one will be on the meter tails and the other, maybe, could go inside the CU?

Trouble is, both inverters are next to each other in the loft and The CU is on the ground floor. (2 Story house).

If both have to be extended to reach, they'll run alongside both inverter's AC cables to the CU.

Maybe we'll wait and see what they do.

Here's the Growatt diagram, I've circled the CT that could stay in the loft, if there's a way.

BTW, there's no DC breaker installed either.

 

[kerching]

Not really bringing anything of value here but, regarding CTs. I have a clamp meter (Megger) designed to work on a twin or 3 core flex or cable

Just saying

 

[binky]

Not really bringing anything of value here but, regarding CTs. I have a clamp meter (Megger) designed to work on a twin or 3 core flex or cable

Just saying

I don't think the CT clamps Norma provided are that sophisticated, but I could be wrong.

 

[johnb2713]

Not really bringing anything of value here but, regarding CTs. I have a clamp meter (Megger) designed to work on a twin or 3 core flex or cable

Just saying

I'd be interested to see details on that, I really don't see how it can work without separating the cores.

 

[michael8554]

That Megger would be for measuring earth leakage ?

I put a CT in a 10mm x 10mm x 5mm box.
Run the T&E through it.
Cut the outer off of about 10mm.
Cut, sleeve, wago the earth, or leave uncut and wrap with insulating tape.

 

[kerching]

I'd be interested to see details on that, I really don't see how it can work without separating the cores.

I'll dig it out later, can't find it on tinterwebnethub

 

[Oldman22]

http://www.precisecalibration.co.uk/product/avo-flexiclamp-200-clamp-meter/
The Megger Flexiclamp does what you always thought clamp meters should do – measure current in multicore cables and power cords without the need to split them.

 

[johnb2713]

http://www.precisecalibration.co.uk/product/avo-flexiclamp-200-clamp-meter/
The Megger Flexiclamp does what you always thought clamp meters should do – measure current in multicore cables and power cords without the need to split them.

Interesting, in theory current flowing up one conductor and back down the other will cancel out the magnetic field needed for a clamp meter to work. The flexiclamp from what I can see uses 4 CT's surrounding the cable, connected as a bridge. The downside of this approach is that it's tuned to frequency and isn't as accurate as conventional CT's. It's convenience is a real plus point though.

 

[kerching]

This is the one I use

https://www.industrialcalibration.co.uk/downloads/MMC850.pdf

 

[johnb2713]

This is the one I use

https://www.industrialcalibration.co.uk/downloads/MMC850.pdf

That spec looks amazing. Every day a school day, today I've learned these things exist, thank you!

 

[kerching]

That spec looks amazing. Every day a school day, today I've learned these things exist, thank you!

It is very good. Ive had it for a few years, bought initially for testing Rack loads on a BFO Dark Site