Dear all,

As a safety measure we have been issued with bench top Isolating transformers. I have measured the voltage between the earth and live at the output of the transformer and found that it is 100 volts. My understanding of these is that the live is no longer referenced to earth and there should be 0 volts P.D.

Does anyone have any experience with these?

Best wishes,

Paul.

Hi Paul,

So your expectation would be that there is 0 volts PD between Earth and Live? Sounds scary!

As you say, the point of an isolating transformer is that it is "isolated" from the main earth. Therefore i would think you cannot reference the output to some random "earth" point, as it has no return path.

Regards,
Robert

Hello Robert,

The rationale of these transformers seems to be that most electrical accidents take place because someone is more likely to be in contact with earth and live than neutral and live. I have measured a voltage of 100v which I accept is a step in the right direction but still think that it is a high enough voltage to cause an accident.

Best wishes

Paul.

The idea of the bench isolating transformer is that the hapless tech can safely (stupidly?) have one paw on an energised "live" part of a piece of equipment and the other paw (or some other extremity) at "earth" ... and still live to tell everyone how dumb he is!

I believe that, technically speaking (and we are, after all, technical folk), it is incorrect to talk about "Live" and "Neutral" in the context of isolating transformers. I should imagine that you'll also see circa. 100 VAC on the other terminal, too, Paul (rather than the near as dammit zero that we hope you find at the Neutral of a normal, earth referenced mains outlet).

As you may imagine, I am not wholly in favour of isolating transformers being used to protect idiots working at benches, mainly because they introduce a non-standard electrical environment (one that the equipment itself will not encounter outside of the operating theatres). They can also lull the tech into an entirely false sense of security. Far better to engage brain before attacking the job at hand. If some kind of safety device is deemed necessary, I suggest the Residual Current Circuit Breaker (sometimes known as a residual-current device).

I have a plug-in RCD outlet adaptor (that I bought cheaply from Wickes) that I carry around for use when working in dodgy (or electrically uncertain) surroundings. It trips out on both poles in less than 30 ms when it detects a imbalance between input and return (due to a fault, or heavy leakage) of 30 mA. Perhaps one day I'll find one that I can adjust to a lower tripping current, but in the meantime the one I have works for me, and I'm not dead yet.

Lastly, isolation transformers sometimes also cause problems for the unwary. Does your electrical safety tester get passed the mains voltage check when plugged in to the iso Tx, for instance?