Connecting an isolation to mains earth

[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.

[televizora]

Re: Connecting an isolation to mains earth

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.

What exactly you will connect to mains earth?
Secondary winding?
The core of the transformer?
The secondary of an insulation transformer must not be connected in any way to mains, because having galvanic connection defeats the whole effect of having insulation transformer in a first place. What makes one conductor Phase and one Neutral in the Mains socket is the fact, that the middle lead of the transformer is connected to Earth in the substation. There is no neutral until the last stage of power transmission. And this is why you can be electrocuted in a first place. Because you can close the connection between Phase/Line and ground anywhere. Because Neutral is usually connected to ground, unless you deal with system with insulated neutral, where it is not connected to the ground.
Most power distribution systems use grounded neutral.
Also, all the equipment that is connected together via interface cables must be connected to the insulation transformer. You cant plug your PC after it and then connect your monitor to the mains, instead to the transformer. This will potentially lead to undesired connection of the secondary winding to the mains and equalization of potentials, which has the power to burn and damage stuff.
If you connect two devises on different mains sockets, that are on different phases, again the equalization of potentials of potentials can burn something. Or if one of the devices has poor ground connection, it will equalize its potential via the one that has good ground connection, via the ground of the interface cables and again this can burn something.

[Nevillet]

Re: Connecting an isolation to mains earth

Many thanks,

The transformer 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 so if a fault develops in the primary winding and it touches the earthed chassis of the transformer it will trip the RCD/GFI.

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?

[budm]

Re: Connecting an isolation to mains earth

You can connect the transformer body to safety ground incase the primary winding is shorted to transformer core, the output winding wires will NOT be connected to any thing but the load so the output will not be reference to safety ground, the ground terminal of the outlet can be connected to safety ground.
That is what we use in our lab and in my test bench.