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Old 27th Jan 2023, 1:04 pm   #44
stuarth
Heptode
 
Join Date: Feb 2010
Location: Heysham, Lancashire, UK.
Posts: 665
Default Re: Isolator Transformer Query

For the case of the "testing machine" described above, isn't the transformer part of the installation? For transformers for specific applications, there are many ways you might want to connect an earth, and hopefully the designer of such equipment understands the issues and designs accordingly.

The discussion here is really about a general purpose transformer on the bench to provide an isolated mains supply, originally for safety on live chassis vintage radio and TV sets, but also for a variety of more modern equipment, often using a SMPS, where we need a fully floating supply, not nessessarily for safety, but to allow unrestricted connection of test equipment anywhere in the circuit. In this case, the transformer should have no fixed earthing on the secondary.

On a closely related subject, the same argument could/should be applied to the earth pin of the output socket on the general purpose isolating transformer, it should not have a fixed earth either. This is to guard against the very dangerous crossed N-E connection mentioned on this forum many times.

I would say the blanket rule might be something along these lines:-

For a transformer in a specific application, the designer should connect an earth to an appropriate place, (eg a point on the transformer secondary, or the earth pin of the transformer output socket to absorb leakage currents from filters).

For a general purpose mains isolation transformer, do not have any fixed earth connection, either to the secondary, or to the output socket on the transformer. The user is then free to connect an earth anywhere at the device under test if required.

Note that an earth does not always improve safety - see the drawing of the N-E crossed connection earlier, or just ask the sparrows on the power line.

Stuart
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