RCD safety for isolation transformer

Thread Starter

Ahmet Pehlivan

Joined Oct 16, 2022
19
I want to use isolation transformer for isolating the osyloscope from the grid circuitry my questions are if i add a rcd relay to the output of isolation transformer how should i wire the earth and does wiring the earth effects the isolation from the grid ?
 

Ian0

Joined Aug 7, 2020
9,764
I want to use isolation transformer for isolating the osyloscope from the grid circuitry my questions are if i add a rcd relay to the output of isolation transformer how should i wire the earth and does wiring the earth effects the isolation from the grid ?
The oscilloscope will be powered from an internal low voltage supply, which will isolate all its internal circuitry from the grid. An additional isolation transformer is pointless.
On the other hand, an isolation transformer to isolate from the grid the equipment you might be testing with the oscilloscope, might be wise.
 

BobTPH

Joined Jun 5, 2013
8,921
The RCD does not need a ground connection to operate. It senses the difference in current between line and neutral. But, with an isolation transformer there is no path back to neutral through a ground, so the currents should never differ.
 

spar59

Joined Aug 4, 2007
64
I want to use isolation transformer for isolating the osyloscope from the grid circuitry my questions are if i add a rcd relay to the output of isolation transformer how should i wire the earth and does wiring the earth effects the isolation from the grid ?
First I am going to assume you mean isolating the mains supply from the oscilloscopes power input, rather than isolating the scope's measuring inputs from the equipment under test.

The output of the isolation transformer would be floating with no reference to ground, hence an RCD positioned at this point is illogical, you might consider one on the transformer input if there is no such device upstream on the mains side.

Note that proper isolation transformers are not just normal transformers with a 1:1 ratio but need to meet specific safety standards regarding the quality of the insulation between primary and secondary and if not wound on separate bobbins should ideally have an inter-winding screen for both safety from insulation failure and to prevent capacitive coupling between primary and secondary.

Now whereas this approach will allow you to float the oscilloscope chassis and avoid earth loops, the scope's chassis will be connected via the screened side of the test lead to the equipment under test, this will allow testing of a voltage, neither side of which is at ground potential but should not be used to test mains voltages as the scope's test leads and chassis could end up being raised to line potential above ground and that is hazardous. To test floating high voltages you need a special high voltage differential probe, these are not cheap but are the safe way to do the test.

An alternative way to do the test, if the scope has at least 2 inputs and the capability to subtract one signal from the other, is with 2 non-differential probes suitable for the voltages being tested, in which case the scope's chassis could remain earthed, each probe is connected to one of the scope's input channels and the probe tips connected to either side of what you wish to measure and the maths function of the scope is used to subtract one signal from the other, thus effectively producing a differential reading.

If I read the question wrong and you intend putting the transformer between the unit under test and the scope's signal input then logically you would use a transforme that gives a safe low voltage output and multiply the readings obtained by the transformer ratio. I accept that this may not be the best solution if you are looking for high frequency distortion of the mains waveform as the transformer will be optimised for mains frequency, the laminations would need to be thinner for good high frequency performance.

Unless it is a transformer designed for instrumentation use you will need to verify the ratio via a voltmeter as for example a 240:24V transformer may not have a ration of 10:1 as it may be tweaked to allow for transformer load regulation.

Again an RCD on the secondary side which is floating from earth is not really logical but one on the feed to the transformer's primary winding may make some sense if there is no such device upstream on the mains side.
 
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