Isolation Transformer Connections

[nevillet]

Hi All,

I have acquired a hefty isolation transformer for use on(under) the bench and have a question about earth bonding.

Apart from the live & neutral connections, should the transformer itself be connected to mains earth?

If so, should the device under test, powered by the IT also be connected to the transformer earth?

Many thanks in advance.

[Zero999]

Hi All,

I have acquired a hefty isolation transformer for use on(under) the bench and have a question about earth bonding.

Apart from the live & neutral connections, should the transformer itself be connected to mains earth?

It depends on the transformer. If it's a toroidal transformer, or the core is double insulated, then it's not necessary to earth it. If the transformer has a screen or the core isn't double insulated, then yes, it should be earthed. Presumably it's a new transformer, in which case it should be earthed, if there's a connection designated to it.

In any case, the secondary winding shouldn't be earthed, because the whole point of a transformer is to separate the secondary from the earth.

If so, should the device under test, powered by the IT also be connected to the transformer earth?

Many thanks in advance.

I would say not, it's not necessary to earth the device connected to the secondary, but doing so wouldn't do any harm.

[nevillet]

Many thanks,

The transformer is not toroidal and just looks like a 'normal' transformer with 2 wires into the primary & 2 wires out of the secondary. 
The laminated former is bolted together and I can easily connect it to the supply earth. 

I will not connect the earthed transformer to the earth of the device under test. 

I see that connecting the device under test to the earth is in effect connecting it to the neutral line.

Have I interpreted this correctly?

[BravoV]

As you mentioned its not toroidal, assuming its EI core like below, make sure you connect the Earth to the transformer body, in this example, tie the Earth wire to the transformer bracket so it connect to the transformer's body.

This is for safety reason, if something wrong with the transformer it self, fault condition will trip the circuit breaker.

[bob91343]

Be careful how you ground the transformer.  It's bolted together, but look closely and you will see that the bolts are insulated with fiber washers.

If you want to connect a ground, do so with the mounting screws so the shell is grounded.

The better transformers will have a ground lead; apparently yours does not.

[joeyjoejoe]

I saw a video reviewing some differential scope probes. The guy was saying to be truly safe you need differential probes AND an isolation transformer.

I assume he's talking about safe with respect to the human bits, and not the DUT or scope, since the differential probes could be poked anywhere without any ill effect?

Having to choose an isolation transformer or differential probes, I think I would prefer the probes. I can keep my fingers out of things.

[bob91343]

It all depends.  If working on a nonisolated unit such as an AA5 (All American 5 tube radio) you need the isolation transformer.  Once you have it in the system feel free to use any old scope probe anywhere, as long as you respect the maximum allowable input voltage.

The differential probe isn't intended to isolate.  It's intended to measure the difference between two points when neither is ground.  This might be across a plate or collector load impedance for instance, or across a high side current sampling resistor.

[Zero999]

As you mentioned its not toroidal, assuming its EI core like below, make sure you connect the Earth to the transformer body, in this example, tie the Earth wire to the transformer bracket so it connect to the transformer's body.

This is for safety reason, if something wrong with the transformer it self, fault condition will trip the circuit breaker.

The transformer in that photograph appears to have double insulated windings, so in theory doesn't need to be earthed. In reality, it'll probably be fitted in a metal enclosure and the connections to the primary aren't double insulated, so the case will need to be earthed, which will also earth the transformer core.

[BravoV]

My point is the Earth must not left flapping and floating on the breeze connected to nothing, as if somehow and something going wrong say like the transformer is overheated and somehow the primary winding gets exposed and electrically connected to the transformer and/or chassis or case, then it will create short to the Earth that will trip the breaker, thats all.

[Zero999]

My point is the Earth must not left flapping and floating on the breeze connected to nothing, as if somehow and something going wrong say like the transformer is overheated and somehow the primary winding gets exposed and electrically connected to the transformer and/or chassis or case, then it will create short to the Earth that will trip the breaker, thats all.

If the transformer overheated, the thermal protection (bimetal strip or thermal fuse) should disconnect it, to prevent a fire.

[BravoV]

My point is the Earth must not left flapping and floating on the breeze connected to nothing, as if somehow and something going wrong say like the transformer is overheated and somehow the primary winding gets exposed and electrically connected to the transformer and/or chassis or case, then it will create short to the Earth that will trip the breaker, thats all.

If the transformer overheated, the thermal protection (bimetal strip or thermal fuse) should disconnect it, to prevent a fire.

Yep, "assuming" there is a fuse, but with these age of cheap stuffs, you just never know. Want to bet your life on that ?

[Zero999]

My point is the Earth must not left flapping and floating on the breeze connected to nothing, as if somehow and something going wrong say like the transformer is overheated and somehow the primary winding gets exposed and electrically connected to the transformer and/or chassis or case, then it will create short to the Earth that will trip the breaker, thats all.

If the transformer overheated, the thermal protection (bimetal strip or thermal fuse) should disconnect it, to prevent a fire.

Yep, "assuming" there is a fuse, but with these age of cheap stuffs, you just never know. Want to bet your life on that ?

No I certainly wouldn't bet my life on it. If it doesn't have a fuse, then it shouldn't be used. Fire is far more of a risk to life and property, than electric shock.

Always add a fuse. Ideally the transformer should have a thermal fuse/bimetal strip built-in, but if not, fuse both the primary and secondary and consider fitting a thermal one.