Step down transformer earthing

[PsychMan]

I have a jukebox (USA made) with an isolating step down transformer inside to convert UK voltage to US voltage. So one side has 240v in and the other 120v out.

The transformer casing is earthed from the mains supply, but I wonder how the rest of the device should be earthed. Presently the Psu and everything else is also earthed from the UK mains feed, but I'm wondering if this is infact serving any purpose as the 120v side doesn't have the same potential?

Is there any guidance on how this should be done? Previously there was an autotransformer installed but I discovered this was delivering insufficient voltage (105v) and I had a isolating transformer spare so thought that might be better anyway in terms of safety

[ukcol]

I would combine the 2 earths into a single protective earth lead.

[PsychMan]

That's kind of what I've done looking at the diagram. My mains cable enters the cabinet, live and neutral go into one side of the transformer, the earth tags onto one of the cheeks of the transformer, then my jukebox power supply and everything else also tags onto where I've bolted the earth. I've got continuity from my earth pin on the plug to all parts that should be earthed, just got thinking as to whether it would still have the same earthing potential as the uk mains when using an isolation transformer.

I'm still learning here so bear with me! As I understand it the benefit of an isolating transformer is that your power doesn't ground in the same way, hence it shouldn't ground to a human being and cause shock. I may have phrased that a little cack handed

[ukcol]

Originally Posted by PsychMan

I'm still learning here so bear with me! As I understand it the benefit of an isolating transformer is that your power doesn't ground in the same way, hence it shouldn't ground to a human being and cause shock. I may have phrased that a little cack handed

The particular advantage of an isolation transformer is that neither phase of its output is connected to earth.

The neutral of the mains supply is connected to earth, at source, and we humans are also connected to earth, albeit through some resistance, by virtue of the fact that we are standing on terra firma. If you are unfortunate enough to touch the live phase conductor a current can flow through you via the Earth back to the neutral side of the power source.

If you touch one phase of the output of an isolation transformer there is no complete circuit path and so you don't get a shock. Things are very different if you touch both phases at once of course.

BTW it seems to me your original auto transformer's output of 105 volts isn't particularly low since the nominal domestic mains voltage in the U.S. is 110 volts.

[Richard]

Hi

Dependant on the type of isolation transformer the output may well have a centre tap connected to earth. If it has just connecting the chassis to earth as in the drawing will be fine.

Richard

[broadgage]

The actual voltage supplied in the USA varies a bit, but the nominal or declared voltage is 120 volts and not 110 volts, so 105 is probably a bit low.

The usual tolerance is plus or minus 5% so that would give a minimum of 114 at the meter, or about 110 volts minimum at the point of use after allowing about 3% voltage drop in the premises wiring.

Some authorities recommend that when 60 cycle American equipment is used via a transformer from UK 50 cycle supplies, that the voltage should be at the lower end of the range, say about 110 volts rather 120 volts.
This is because transformers, relays, lamp ballasts and other magnetic components designed for 60 cycles might draw excessive current if subjected to full voltage at 50 cycles.

[Al (astral highway)]

Originally Posted by ukcol

If you touch one phase of the output of an isolation transformer there is no complete circuit path and so you don't get a shock. Things are very different if you touch both phases at once of course...

Yes...except... if the transformer looks like an isolating transformer without actually being one.

Eg, one side of the secondary of all scrapped microwave oven transformers I've seen is internally grounded to the case, where it forms part of the circuit with the anode of the magnetron and the grounded cathode of the voltage doubler diode.

But they look exactly like isolation transformers until you inspect them closely, and still, as a potential source of confusion, have two terminals on the secondary side.

Touching the non-grounded side, potential 2,000V (that's before it goes through the voltage doubler to the magnetron) is one-touch lethal ...these things can source 1A continuously.

I use this example not to focus on this specific application, but to suggest that it's always worth checking whether a pulled 'isolation transformer' really is what it appears to be.

[PsychMan]

well that's certainly a lot to think about, not sure I understand it all in honesty!

The transformer I installed was an original pulled out of another "export" machine, so I would imagine the voltage was recommended by the manufacturer. I've noticed my mechanism motors running a bit faster, so the carousel spins faster as does the gripper mech. Before it was a little sluggish in operation

Regarding whether this is a real isolating transformer, are there any simple checks I can do? The only terminals for connection were 2 on each side (UK / US), nowhere to attach an earth which is why I added it to the transformer chassis. I noticed no continuity between both windings

Just really looking to confirm what I've done is relatively safe - I did open the transformer and check the internal wire insulation before using it, and was surprised to find it in perfect condition.

[Al (astral highway)]

Hi, I'm not trying to complicate what's probably a very straightforward thing with what you're doing. But it's a cautionary tale, if you like...no room whatsoever for error in the case I've described.

I'd also add that the best isolating transformers have a centre-tap, which is grounded. That exactly halves the voltage available to do any harm in the event of a shock.

[AC/HL]

Although the transformer has an isolated secondary, it's only being used as a step-down transformer in this case, as was it's predecessor.
Post two would seem to be perfectly adequate.
Mains isolation only becomes an issue with external or AC/DC equipment.

[PsychMan]

OK so seems like I've done the right thing by earthing the transformer chassis and the jukebox PSU from the mains socket

Thanks all for the advice, I'll be reading it again and trying to get my head round it.

Ideally in the future I'd get a better transformer, but so far this project has cost me a fortune so I'll make do with the old transformer and strict earthing. I got it along with a huge batch of parts for this machine so pretty much cost me nothing and is in nice serviceable condition

[Herald1360]

Originally Posted by astral highway

I'd also add that the best isolating transformers have a centre-tap, which is grounded. That exactly halves the voltage available to do any harm in the event of a shock.

If the secondary centre tap is grounded, where's the isolation? One side of the mains is nominally grounded too. Isolation suggests that there is no galvanic connection between primary and secondary

That sounds like a "site" transformer which is intended to supply 110V to power tools etc but only expose the operator to 55V max which is relatively safe compared with 240V. It's a special case of a transformer for a specific purpose rather than a generic isolation type.

Getting back to the Jukebox- presumably that's fitted with its own internal 110V mains transformer which would have allowed the chassis etc to be grounded anyway. If the stepdown transformer is regarded as part of the mains supply rather than part of the jukebox then it doesn't matter whether it's an isolating type or an auto type since its output 110v would be no different from the original 110V from US mains and should be insulated accordingly.

50/60Hz- a transformer designed on the limit for 60Hz will run hot at 50Hz since the primary magnetising current will be 20% higher at least. It could even saturate the core on current peaks in which case it would be very unhappy!