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Old 28th Jan 2023, 9:34 am   #49
julie_m
Dekatron
 
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
Default Re: Isolator Transformer Query

The real problem is, there are two quite distinct reasons why you might want a piece of equipment to be supplied by an isolating transformer.

Scenario 1: You want to avoid creating a dangerous situation if any exposed metalwork on the appliance should come into contact with one side of the power supply. This might happen using an electric shaver in a bathroom, or an unknown guitar amplifier in a TV studio, for instance. In this case, the secondary needs not to be Earthed, and appropriate care should be taken to minimise parasitic coupling to Earth: keep all connections short and don't have any "real" Earth connection to the appliance (even the capacitance between adjacent conductors in a mains lead may let enough current flow to be noticed as a "tingle"). The deliberately-introduced, very high impedance between the appliance supply and Earth will limit the current in case any fault brings the user (assumed to have a low-impedance Earth connection) into contact with either terminal of the appliance supply.

Scenario 2: You want a "clean" Earth reference for test equipment connected to a live-chassis appliance, excluding the building wiring back to wherever the Neutral and Earth are joined (usually right next to the company head, in an urban area). In this scenario, one end of the secondary is Earthed, and so is the test equipment (oscilloscope, signal generator, Arduino connected to PC, or similar). Any safety advantage of a floating secondary is negated by the Earth connection, but it means the appliance's chassis is connected directly to a common Earth with the test equipment; and specifically, not in series with some unknown impedance which is also carrying a varying current to other, unrelated appliances in the building.

The real danger is thinking that an isolating transformer is magic. Just because it can sometimes offer protection from a dangerous situation, doesn't mean it can't be used in a way that won't offer that protection.
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