Isolation Traffo/Generator.

Tuco88

Hi Lads,

We recently carried out a check on a few sockets in a medical setup, which are feed from a 1:1 Isolation Traffo. The Sockets had an Earth in place (I thought the Earth was not need due to Isolation traffo?) We are Getting ~115v from Live to Earth, I take this is from a floating Earth due to the Earth in the socket going back to the main building Earth bar. There is also ~115v between Neutral and Earth this is what has me a little worried is this correct should the Earths be removed will it effect RCD operation?


We have a new Generator to install also is a 8kva not in place yet, I had a look at the output voltages are 230v L-N but its 115v~ between N-E is this normal? I cant say iv had this issue before.

Any views id love to hear them,

Cheers.

2011

Tuco88 wrote: »

We recently carried out a check on a few sockets in a medical setup, which are feed from a 1:1 Isolation Traffo. The Sockets had an Earth in place (I thought the Earth was not need due to Isolation traffo?) We are Getting ~115v from Live to Earth, I take this is from a floating Earth due to the Earth in the socket going back to the main building Earth bar.

No, mosy likely with secondry side of the transformer is center tapped. Therefore the supply to the sockets is 2 phase with 230 volts between the 2 phases.

You no longer have a neutral that is neutralized (connected to earth).

This means that the maximum shock that a person can recieve (unless they are across both phases) is 115V.

2011

Just to add to the above, because the "neutral" is in reality a phase the protective devices (MCB's / Fuses / RCBOs) installed should be 2 pole as both phases are equally live with respect to earth.

To answer your other question, RCBO's should work fine once the circuit is healthy.

Tuco88

Thanks 2011,

Your on the ball as usual

That's what I had in my mind the same as a 110v site transformer lets say 55v etc.... But the large transformer under the (panel supplying it) has "Isolation Transformer" label on it, should it not be the C.T.E label instead so?. If my understanding is correct an Isolation Transformer Secondary L+N should not have any reference to Earth?

Tuco88

Tuco88 wrote: »

If my understanding is correct an Isolation Transformer Secondary L+N should not have any reference to Earth?

Yurp.
You can use a floating earth for circuit protection but it's not isolated if you use the supply earth.

Bruthal

Yea its centre tapped secondary side. No actual neutral exists. 115v from each pole to the centre tapped earth. Double pole mcbs. Rcbos will work. Although if they were 30ma ones, they are unlikely to ever trip due to a person receiving a shock. But can protect from fire etc in the event of earth faults.

It's still fully isolated once the centre tapped earth is not connected to supply earth.

2011

Bruthal wrote: »

It's still fully isolated once the centre tapped earth is not connected to supply earth.

Perhaps these earths are not connected.
Have you checked this Tuco88 ??

gctest50

Tuco88 wrote: »

............................We have a new Generator to install also is a 8kva not in place yet...........

a UPS would be nice to have on it too, tide it over until the generator starts up and sorts itself out