Isolation transformer refit - worth proceeding?

[skyhawk]

Hi guys,

Per this thread I got myself an old Hammond 171E isolation transformer off Kijiji, intending to use it as a lab isolator for protecting myself and my shiny new oscilloscope. This unit differs from the unit I linked to in that it has no switch or over-current protection device, and has a duplex receptacle on the secondary side rather than a short tail.

I just got the thing apart intending to figure out where and how to mount a breaker and switch, and holy oh my god this thing looks and feels crusty. Prior to disassembly it buzzed like a bastard (but did work) when plugged in. I can't find a manufacture date anywhere on it. I suspect it may be older than I am. The lamination material is hard as rock, there's no give and I feel like I'm going to start breaking pieces off every time I touch it. Based on the amount of paint that came off I'm the first person to have disassembled this thing since it left the factory.

I have a feeling I should write this thing off as a mistake rather than spend even a single additional penny on it. I'm looking for opinions from people more familiar with the construction and failure modes of transformers than I am. What do you think?

[tautech]

If the lamination/cover bolts were loose it might hum louder. But they do hum anyway.
Looks just like a fine old school transformer. Handle gently.

If you know an electrician, get a leakage test done with a Megger or insulation tester....just for piece of mind.

Consider a surface mount box for the socket and switching.
Then you might have sufficent space also for a "push button" type breaker.

[nctnico]

Don't use an isolation transformer! There are too many things which can go wrong (thinking you are safe is one of them) and create an even more dangerous situation.
Spend money on a proper differential probe and a sensitive GFI.

[richard.cs]

The transformer looks fine, they last pretty much forever if they're not overloaded. I'd be inclined to add overcurrent protection to the output but other than that I'd leave it as it is.

Do make sure you understand what an isolation transformer is and isn't good for, in particular that once you hang a grounded scope off your DUT you've lost isolation again. That's normal and fine just don't go expecting it to be safe to touch.

[dom0]

This forum actually has a ignore user function. Engineering marvel!

[nctnico]

This forum actually has a ignore user function. Engineering marvel!

Well go ahead and kill yourself in absence of life saving advice!

Do make sure you understand what an isolation transformer is and isn't good for, in particular that once you hang a grounded scope off your DUT you've lost isolation again. That's normal and fine just don't go expecting it to be safe to touch.

That's normal and fine just don't go expecting it to be safe to touch 

[richard.cs]

An isolation transformer allows you to take a smps where the 0V reference for the circuit (the bridge rectifier output) would normally bounce up and down by the full mains voltage and tie that point to ground. Having done so it becomes possible to make all kinds of measurements that are otherwise difficult to do. Changing the ground reference is a useful thing to do but doesn't improve safety - so what? That doesn't make the isolation transformer valueless.

An isolation transformer can also be used to improve safety in a fully floating system if you are careful to ensure it stays floating. It's safer because two faults rather than one are necessary to pass a dangerous current through the user. That's an entirely different use and also perfectly legitimate.

My point was that beginners often conflate these two uses which are fundementally incompatible. You can't do the first one and expect to see the safety benefits of the second.  Indeed you don't get any safety benefits from the second one without being very careful in what you connect to it because the "first fault" doesn't have any obvious effect.