Electrical testing

[Chilty96]

We are carrying out some electrical testing on an installation. From the fuse board it supplies 415v to a transformer which is stepped down to 230v. From there it is wired into a panel via 3 phases a neutral and an earth. When carrying out the Zs test we got 1.7 ohms on L1 and then 50 ohms on L2 and 3 why is is this happening any help would be appreciated

 

[timhoward]

I think you are describing a 3 phase 3 wire supply feeding a 3 phase isolating transformer?
Is it just 3 phases going to the transformer, and 3 phases and N coming out?
Key question - what is the earthing both sides of the transformer?
Does the load connected to this transformer have an earthed case?

(I guess another possibility is it's purpose is to avoid high frequency noise being back-fed into the supply.)
Knowing what it is there for and designed to do is rather essential before diving into the testing really.
I might be barking up wrong tree but one thought process is if the transformer is to provide an IT supply maybe the outer case should cause ADS if if became live, and the secondary earthing might be connected to a rectifier to achieve this, hence the different readings.

In the meantime obviously with supply disconnected I'd do dead R1+R2 tests for each phase on the primary side, and measure between phases, and to N, and to E on the secondary side to help build a picture of what is going on.
All that to say, this is the kind of territory where extra care is needed as it isn't 'typical'. More information about it will help.

( @pc1966 , @Lucien Nunes this might be one for you guys )

 

[Julie.]

As above, the actual winding arrangements of the transformer are key to understanding the measurements you should expect.

If it is a typical Dyn arrangement - providing earth isolation then yes I would expect all three phases to be the same at a low value.

However without understanding the use/winding arrangement this is a guess, there are many circumstances where a special transformer is required resulting in low resistance in one winding, but high in others.

 

[pc1966]

As above, can you provide more information on the transformer, etc? Perhaps some photos including any rating/details plate on the transformer itself.

I would agree with @timhoward that (after safe isolation) you try some DC R1+R2 tests to identify if the reason for 50 ohms is an inductive term or it really is restive (which would surprise me to be fair, as I can't see how it would share a common neutral/earth with L1 in this situation).

 

[Chilty96]

I think you are describing a 3 phase 3 wire supply feeding a 3 phase isolating transformer?
Is it just 3 phases going to the transformer, and 3 phases and N coming out?
Key question - what is the earthing both sides of the transformer?
Does the load connected to this transformer have an earthed case?

(I guess another possibility is it's purpose is to avoid high frequency noise being back-fed into the supply.)
Knowing what it is there for and designed to do is rather essential before diving into the testing really.
I might be barking up wrong tree but one thought process is if the transformer is to provide an IT supply maybe the outer case should cause ADS if if became live, and the secondary earthing might be connected to a rectifier to achieve this, hence the different readings.

In the meantime obviously with supply disconnected I'd do dead R1+R2 tests for each phase on the primary side, and measure between phases, and to N, and to E on the secondary side to help build a picture of what is going on.
All that to say, this is the kind of territory where extra care is needed as it isn't 'typical'. More information about it will help.

( @pc1966 , @Lucien Nunes this might be one for you guys )

So the supply is 3 phases L1, L2 and L3 with a neutral and an earth. And the same for the load going out. The load does have an earthed case. We carried out dead checks from by placing a link at the supplies and testing on the load and we got good results

 

[Julie.]

So the supply is 3 phases L1, L2 and L3 with a neutral and an earth. And the same for the load going out. The load does have an earthed case. We carried out dead checks from by placing a link at the supplies and testing on the load and we got good results

We need to know how the transformer windings are connected.

Incoming, are all three phases and neutral connected to windings?

If so why the transformer at all.

 

[Chilty96]

So the supply is 3 phases L1, L2 and L3 with a neutral and an earth. And the same for the load going out. The load does have an earthed case. We carried out dead checks from by placing a link at the supplies and testing on the load and we got good results

We need to know how the transformer windings are connected.

Incoming, are all three phases and neutral connected to windings?

If so why the transformer at all.

We need to know how the transformer windings are connected.

Incoming, are all three phases and neutral connected to windings?

If so why the transformer at all.

Only 3 phases in Delta

 

[Julie.]

OK, assuming the secondary is star, bringing the star point out as neutral and earth, I would expect all phases to measure the same rs/re - have you tested right at the transformer terminals?

Unless the secondary is some special winding, they should all be the same