Seeking advice on mains equipment earthing

I'm restoring a 1930s film editor for a friend. Electrically it is very simple, just a transformer with a 24 volt output for the bulb. However, it has a metal case and only a 2-core mains lead, which makes me very uncomfortable.

The obvious solution is to fit a 3-core mains lead and connect the earth wire to the case. However, the secondary (bulb) circuit already uses the case as a return path, and it wouldn't be easy to replace it with a wired connection.

My question is this: is there any reason for not earthing the case, and thereby one leg of the transformer secondary? I cannot think of any such reason, but those with more experience than me may know something I don't.

George

Edited By Georgineer on 25/02/2022 21:38:26

Assuming that the bulb circuit is a proper isolated secondary, i.e. it's not an auto-transformer, there should not be a problem, in fact it would be recommended. Given the age of the transformer the inter-winding insulation could be a bit dodgy by now, which is a good reason to earth the metalwork!

George

Most likely the safest option is to remove the 240/24v transformer from the editor and mount it in a small enclosure wirh a supply via a suitably fused 13A plug and 3 core cable so the metal of the transformer can be earthed.
Then wire the 24v supply only to the editor and connect to the same place where the transformer secondary was terminated.
As John has already said beware if it is an auto transformer, if that is the case best get a new double wound transformer so there is no chance of mains voltage gong to the editor.

Emgee

No reason not to ground the case. It is the best pln. The fact that the 24V lamp also uses chassis ground is not an issue. It's common practice. It is highly unlikely that the existing transformer is an auto-transformer. There is no advantge for a high current step-down trnsformer to be an auto-transformer.
A simple continuity check from the two pins of the mains plug (conneted together) will tell you. If there is continuity there is an issue. This is unlikely though. Ideally you would also do an insulation check with megger.

Just replace the cord with a 3 conductor one and connect the green/yellow wire to the chassis. Don't forget to use a smaller fuse (3 or 5 Amp) in the plug.

Robert G8RPI.

Edited By Robert Atkinson 2 on 26/02/2022 21:45:54

Thanks for your replies. I'm glad my initial thinking was on the right lines.

I don't believe it could be fitted with an auto-transformer, because that would have connected the metal case either directly to the live, or through a low resistance winding. Emgee, I don't think there is any reason to remove the transformer to a separate housing.

The finished article will be PAT tested when I've finished, which will show up any inadequacies in insulation resistance.

I wonder how many of our readers are aware that the latest edition of the IET code of practice has removed all reference to appliances being 'portable'. My suspicion is that the world will carry on referring to PAT testing, just as it has to the MOT test, decades after the Ministry of Transport ceased to exist.

George

Unless you are restoring the item as a historical item and trying to keep it as near to original as possible (in which case earthing the case would be appropriate) I would replace the mains input with a 24V feed from a modern plug in power supply and bypass the transformer entirely.

regards Martin

As transformers are robust unless overheated or stored in the damp, I wouldn't replace it unless it failed a test.

To keep costs down older equipment often left out an internal fuse, power on indicator and switch. I might add one or more, but not if it spoilt the appearance. It is worth putting the smallest possible fuse in the mains plug. Although 3A, 5A and 13A are the most common UK plug fuses, you can also get 1A. (and 2A, 7A & 10A. 7A may be a collectors item I've never seen one.)

If the film editor already has an indicator AND a switch AND the switch is a double pole type in this circuit it may be worth replacing the switch with a single pole type or otherwise eliminating the switched neutral (red ringed).

The safety argument on this issue see-saws! Here's why it can be dangerous to switch neutral. If the switch fails with the upper contacts stuck together whilst the lower switch opens normally then the indicator 'I' goes out suggesting the apparatus is safe when it's actually still live. In practice, diagnosticians were rarely caught out but a few got zapped. Personally I don't believe power electricity is safe unless the device is physically unplugged and I'm certain some passing numpty can't reconnect it while I'm working inside! Electricity is much safer if you don't assume anything : make sure it's not live before touching it!

Dave

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Dave,

The switch is a single pole type, which I find a little surprising as the original mains lead has a moulded-on reversible 2-pin continental plug. No need for an indicator in this application, as the main bulb does that job. No fuse, though I note that later versions of the same machine did have one. I might well fit one.

A quick test with a multimeter indicates greater than 20 meg resistance between primary and secondary. My PAT tester will test it at 500 V before it is put back in service.

br,

I've got a 7 A fuse too, and a number of other historical oddities. I think this could become the collecting craze of the future. Swap your 10 A yellow for my 3 A blue?

George

Edit: Correction; it was a wired-on plug, not moulded-on, but still reversible

Edited By Georgineer on 27/02/2022 12:14:43

Hi, plenty of 7A plug top fuses and other types here Dave CPC UK

Regards Nick.

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Yes, The fuse in a mains plug is there to protect the cable from the plug to the device from overheating or melting, so the fuse needs to be rated for the cable size, not the plug capacity. Not many plug connected household appliances draw 13A, which is about 3 kW ! But too many small devices are, or used to be, supplied with a 13A fuse.

My understanding is that unless a device is double insulated, (and shows the double square logo), any external metalwork should, ideally, be connected to earth.

The earth lead is there to carry sufficient current to blow the fuse in the plug if the metalwork becomes live, thus disconnecting the device from the supply.

Edited By John Doe 2 on 20/04/2022 12:07:12

I'm pleased to report back that the restoration has been successful. What took the time was getting my hands on a suitable bulb for a reasonable price. The previous owner had fitted a 12 volt car indicator bulb, which got a bit over-excited on 24 volts.

I concluded that there is no need to fit a fuse in the editor itself. I take John Doe's point that officially the fuse in the plug is there to protect the flexible cable not the equipment itself, but with a 3 amp fuse fitted in the plug it will blow long before anything else comes to any harm.

A 20-amp class 1 PAT test shows 0.03 ohm resistance (± tolerance) between metal casework and earth pin which is good, and insulation resistance between transformer windings >20 megohms, which is even gooder. My work here is done!

Thanks again to all who have contributed,

George

Hi there, George,

I'm sorry to chime in when I'm so late to the party.

What none of the previous posters have mentioned is the clamp where the cable enters the editor. Obviously you have changed the cable to upgrade from two cores to three but did that allow the previous cable clamp to be reused?

It's essential that any tension (e.g. an accidental tug or trip ) on the cable is transmitted to the editor case rather than ripping out the internal connections by the roots!!

Best regards,

Swarf, Mostly!

Yes, I'm pleased to say that it did. The three-core flex was chosen to be the same diameter as the original two-core, so I was able to re-use the original anchoring arrangements.

George

Something I was taught many years ago is to make the earth wire longer than the 'live' wire inside a plug or a machine connection.

So in a 13A plug I make the 'live' wire just long enough to reach the live terminal but I make the earth wire longer than it needs to be with a loop or a fold so if the whole lead got pulled, it would tear out the 'live', leaving the earth wire still connected. Or if the lead was completely ripped out, the earth would be the last conductor to be disconnected.

This sort of follows on from properly designed mains plugs in which the earth conductor is the first to make contact when plugging in and the last to disconnect when unplugging - owing to the earth pin being longer than the other pins.

Edited By John Doe 2 on 22/04/2022 10:17:18

Something I've been doing since Dad taught me how to wire a plug in nineteen-sixty-... er... is that the time?

I did the same inside the film editor, for the same reasons.

George