11-02-2015, 12:03 AM
An isolation transformer is purely to allow the connection of test equipment to a device under test.
As soon as the earth clip of a 'scope probe (for example) has made contact with a point in the circuit, everything else in the DUT has a potential that is relative to mains earth.
If there are multiple devices plugged into the isolation transformer, then those devices are no longer floating with respect to mains earth. Every part of them will either be at earth potential or some other potential with respect to mains earth. That means that you are at risk of an electric shock - perhaps just from touching the case of another item of gear if you were especially unlucky. It also means that you probably can't connect some other item of test gear that has an earth reference (e.g. a sig gen) while your 'scope is still connected to the first item.
Further to this, most people IME are not aware that the mains earth must not be carried through to the (singular!) BS1363 outlet of your isolation transformer. There have been instances where electric shocks have been received by simply touching the metal case of a DUT, owing to a N-E swap in the mains plug. A chunky 4mm binding post connected to mains earth should be provided, so the operator can firmly earth any part of the DUT as required.
I've attached a BBC guidance note about isolation transformers, which hopefully will be interesting reading to everyone here. Note the mention of RCDs. These generally offer greater safety on a workbench, and should be the default way to supply gear when "working live".
As soon as the earth clip of a 'scope probe (for example) has made contact with a point in the circuit, everything else in the DUT has a potential that is relative to mains earth.
If there are multiple devices plugged into the isolation transformer, then those devices are no longer floating with respect to mains earth. Every part of them will either be at earth potential or some other potential with respect to mains earth. That means that you are at risk of an electric shock - perhaps just from touching the case of another item of gear if you were especially unlucky. It also means that you probably can't connect some other item of test gear that has an earth reference (e.g. a sig gen) while your 'scope is still connected to the first item.
Further to this, most people IME are not aware that the mains earth must not be carried through to the (singular!) BS1363 outlet of your isolation transformer. There have been instances where electric shocks have been received by simply touching the metal case of a DUT, owing to a N-E swap in the mains plug. A chunky 4mm binding post connected to mains earth should be provided, so the operator can firmly earth any part of the DUT as required.
I've attached a BBC guidance note about isolation transformers, which hopefully will be interesting reading to everyone here. Note the mention of RCDs. These generally offer greater safety on a workbench, and should be the default way to supply gear when "working live".
