UK Eddy currents??

[LouDog]

After a bit of advice. Have a 630A MCCB feeding a DB, it's fed with 4 x 300mm single core, non armoured HO7 cables and a 185mm earth. The DB is GRP and plastic compression glands have been used. The main panel has an aluminium gland plate and brass A2 glands have been used, however, the lock nuts are steel. Will this cause issues, do the lock rings need to be changed to brass also?

 

[stevethesparks]

If the gland plate is aluminium, then it should be fine. If it had been steel, then yes brass locknuts would also have been required along with a slot between gland entries.

 

[Andy-1960]

Any metal that completely surrounds a single core cable should be non-ferrous, that includes lock nuts, so in my opinion these should be changed to brass.

 

[westward10]

Agree with @Andy-1960 they should be non-ferrous.

 

[loz2754]

Any metal that completely surrounds a single core cable should be non-ferrous, that includes lock nuts, so in my opinion these should be changed to brass.

Interesting. I hadn't thought of locknuts being subject to eddy currents, but what you say makes sense when I think back to basic AC circuit theory.

 

[LouDog]

After a bit of advice. Have a 630A MCCB feeding a DB, it's fed with 4 x 300mm single core, non armoured HO7 cables and a 185mm earth. The DB is GRP and plastic compression glands have been used. The main panel has an aluminium gland plate and brass A2 glands have been used, however, the lock nuts are steel. Will this cause issues, do the lock rings need to be changed to brass also?

Any metal that completely surrounds a single core cable should be non-ferrous, that includes lock nuts, so in my opinion these should be changed to brass.

I have always kept the lock nut the same material as the gland on single core cable, just something I was told years ago but never actually fully knew why. The reason I asked is because they are going to be pig to take back off and swap and wondered whether anyone had any experience of it.

 

[Andy-1960]

I have always kept the lock nut the same material as the gland on single core cable, just something I was told years ago but never actually fully knew why. The reason I asked is because they are going to be pig to take back off and swap and wondered whether anyone had any experience of it.

I’m afraid that’s what needs to be done, especially on high current circuits such as this.

 

[westward10]

It will be easier to replace the lock nut on singles, assuming there is ready access than a multi-core cable.

 

[LouDog]

It will be easier to replace the lock nut on singles, assuming there is ready access than a multi-core cable.

I agree, multi core would be worse. There is however a plastic gland plate half way down the enclosure which cables run through an additional plastic compression gland for support. The lug probably won't fit through the plastic compression gland so would have to cut the lug off slide it back down through the plastic gland and then swap the lock nuts.

Nowhere near impossible just a lot of extra work, hence wanting a bit of clarity as people have different opinions on it. Would ideally like to leave the steel ones place but obviously wouldn't if its going to be dangerous.

 

[pc1966]

Is it already in use / under normal load?

If so just check to see if the nuts appear noticeably hotter than the cables.

 

[LouDog]

Is it already in use / under normal load?

If so just check to see if the nuts appear noticeably hotter than the cables.

It is, its suppling a PV system which appears to have been installed late last year. At the time I was there it was cloudy so not much current was flowing back through. Guessing when summer comes round these cables will get hotter as more current flows back, which will heat the lock nuts? Wasnt sure whether the actual glands being brass, gland plate being aluminium and cable being non armoured would have any bearing on potential eddy currents.

 

[westward10]

If the lock nuts are only holding the gland in place and there is no undue force on them from the cables you could undo the nuts and carefully put a cut through them and hope they thread back on.

 

[loz2754]

If the lock nuts are only holding the gland in place and there is no undue force on them from the cables you could undo the nuts and carefully put a cut through them and hope they thread back on.

I thought of that too. But somehow I doubt they would hold IMHO.

 

[westward10]

I thought of that too. But somehow I doubt they would hold IMHO.

Depends on how tight they need to hold the gland but I agree they will never be tight. If they only need to hold the gland in place to support the cable entry you could run some Loctite thread sealer into them.

 

[LouDog]

Depends on how tight they need to hold the gland but I agree they will never be tight. If they only need to hold the gland in place to support the cable entry you could run some Loctite thread sealer into them.

 

[LouDog]

Good shout with the slot in the lock ring, might give it a test and see how it holds up

 

[LouDog]

Thanks for the input over the weekend. Back out today, tested the slot method, good quality locknuts and they tightened nicely. Still slightly unsure about the whole thing and whether it's necessary, is the only concern the locknuts getting warm?

 

[westward10]

Yes there is a possibility of heat being generated.

 

[pc1966]

Overheating is really the #1 concern.

It can slightly increase the loop impedance, but that would be taken in to account when measured. Also for very high fault currents the ferromagnetic material is going to saturate anyway, so any inductance would drop to more-or-less the free space values.

Incidentally that is one big PV install if you have a 630A 3-phase connection!

 

[LouDog]

Around the 400kW mark.