SWA from transformer to DB

[stef]

Is this correct: a SWA cable runs from a 3 phase transformer to a distribution board (TNS). The SWA must only
be connected to earth on the transformer side to earth and not on the DB side otherwise you'd get a
second earth loop. But what harm can that do?

 

[mogga]

connect both sides if you have used 4 core spare as earth also BAD Practice in prmanent instals but as the tranny will be moved and inadvertantly knocked by builders leading to probable loosening of the gland IMO put an earth clamp on the armour at either end and connect to earth as I say its viewed as bad practice but due to conditions Id recomed that

 

[stef]

Uhm, I forgot to mention: it is a 500 KVA transformer (11KV to 415V) which was installed by some DNO
contractors. I found it a bit odd why they connected one end only and when I asked them they mentioned the
second earth loop but didn't know exactly why.

 

[benji]

Are you using single core cables ? you don,t mention the neutral ?

yours benji

 

[stef]

Hi benji,

they were using 3 single core SWA per phase, don't know what size. There is a neutral as well.
I really don't know why the SWA is only earthed on one side. The contractors didn't know either, except 'it is always done like this'

 

[mogga]

I was talking about 400>110v tranny m8 not HV soz

 

[Tunnel Sparkie]

Are both ends glanded onto steel gland plates? If so you'll have a parallel loop any way.

 

[stef]

No, they used glands on the transformer side only, on the DB side they used insulated clamps, clamping the cables on the insulating PVC sheath to the frame, (in other words: insulated from earth) and they used a separate earth conductor from the transformer frame and star point to the DB frame. I don't know, it looks a bit strange to me. I googled up and down but couldn't find an explaination for this
Shame that I couldn't take a photo.

 

[davelerave]

Are both ends glanded onto steel gland plates? If so you'll have a parallel loop any way.

think that's it
if you gland the tn-s at the DB and the supply cable is damaged there will be a second fault loop through the gland at the DB back through the earth core on the tn-s supply

 

[stef]

Cheers Dave,

yes that makes sense.

 

[benji]

normally use single cores with aluminium armour glanded both ends into non-ferrous gland plate at one end, think it,s to stop circulating current being induced into armour .how big was the db

yours benji

 

[stef]

1200 Amps mate. They were using aluminium conductors for phases and neutral, earth in copper conductor.

 

[Knobhead]

Sorry this is a bit late.

Armoured transformer tails are unusual, normally double insulated un-armoured. The reason for glanding just one end is to stop the armour wires becoming a giant 1/1 CT. At full load your 500KVA transformer would have 666A flowing in the armours.

I once had a situation where 1000KVA transformer was installed and the contractor decided that the brass gland nuts we’d provided were to nice to use so substituted steel, it was in a bolted terminal box so who would know. Things were OK for a while but as the load on the transformer grew so did the heat. I’m just glad I wasn’t around when the insulation finally melted in the transformer terminal box

The contracting company wasn’t around for long after that!

 

[johnboy6083]

so if 1 end of the AWA is glanded into the ali gland plate, what happens at the other end? is it glanded into one of them insulating bushes, which is then glanded into the gland plate? in was on a power station and they all used AWA singles for their trannys, but i only glanded one end, and we were told by the engineer to just use ali glands straight into gland plate.

cheers mate!

 

[Knobhead]

I was referring to steel wire armour, aluminium can’t set up eddy currents, which cause the problems. The gland plates must have been aluminium, or steel with a slot between each hole. The slot is usually filled with epoxy resin before the plate is sprayed.
Earthing both ends of the tail armouring also causes problems with the restricted earth fault protection relays used on the LV side of a transformer.

 

[johnboy6083]

im a numpty, if course you cant induct into a non ferrous metal. hence te AWA. I wasnt aware that you can get SWA singles for the reason you mentioned.

 

[Knobhead]

im a numpty, if course you cant induct into a non ferrous metal. hence te AWA. I wasnt aware that you can get SWA singles for the reason you mentioned.

Neither was I, all the transformers I’ve worked on had either double insulated or lead sheathed tails. Aluminium never crossed my mind!


The thread started with SWA armoured singles!

 

[davelerave]

Neither was I, all the transformers I’ve worked on had either double insulated or lead sheathed tails. Aluminium never crossed my mind!


The thread started with SWA armoured singles!

are there any issues with earthing both ends of swa in general

when does this become a consideration?

 

[benji]

no problems if you use non-ferous gland plates

yours benji

 

[Alpha Spark]

Another reason could be, that with the armour's connected, the ZE will be tiny. Less that 0.01ohm. This will lead to a massive PFC.

 

[davelerave]

Sorry this is a bit late.

Armoured transformer tails are unusual, normally double insulated un-armoured. The reason for glanding just one end is to stop the armour wires becoming a giant 1/1 CT. At full load your 500KVA transformer would have 666A flowing in the armours.

I once had a situation where 1000KVA transformer was installed and the contractor decided that the brass gland nuts we’d provided were to nice to use so substituted steel, it was in a bolted terminal box so who would know. Things were OK for a while but as the load on the transformer grew so did the heat. I’m just glad I wasn’t around when the insulation finally melted in the transformer terminal box

The contracting company wasn’t around for long after that!

still don't quite get this- are eddys currents an issue on swa's generally

SP and 3P if 'banjoed and flylead' both ends

ie: cpc and armour paralleled


or is it just singles ?
thanks for any info

 

[Knobhead]

With aluminium armouring or lead sheathing one end only will be earthed to stop circulating currents. I went back through my old apprentice training books to find out why we didn’t use aluminium armouring, all the plants produced or processed lime. Aluminium and lime isn’t a good mix, the lime rots the aluminium away, and for the same reason aluminium ladders weren’t allowed. Lead has to be plumbed to terminate it so the cores can be sealed with compound. Each plumbing cone at one end would be insulated from the terminal box. From what I’ve found out aluminium can be glanded at each end but one end like the lead must be insulated. On all the systems using lead the transformer end would be earthed while the switchgear end would be insulated.
Some of the transformers were installed with aluminium conductors but the terminations had to be protected from the lime with a horrible spray on yellow gunk that for the life of me I can’t remember what it was called.
My comment about eddy currents referred to an episode where a contractor used steel locknuts on double insulated singles instead of the brass he was supplied with. The gland plate was steel but with slots and the glands brass to stop the eddy currents, he f*^ked it up by pocketing the brass locknuts.