Voltage from chassis to earth

[Andrewausfa]

I'm currently restoring a very old signal generator. Having replaced the two electrolytic caps I've applied power and everything seems fine. I'd checked for dodgy wiring etc and tested the chassis, as I always do, with a neon tester. This set does not have a factory fitted earth connection and I hadn't fitted one at the time of testing.

I then went to check the output on a 'scope, actually this was the second time I checked the waveforms but...on connecting the scope probes ground lead I noticed it was drawing noticeable sparks as I tried to connect it. Obviously it took me back for a few seconds as I thought the chassis was somehow live. This sparking stopped when I earthed the chassis.

I've Meggered mains TX primary to TX clamps and chassis at 500v, tested at 100+M Ohm. I've connected a meter between chassis and mains earth and this read half mains voltage at around 122v AC which I've read is normal (is it?).

Could someone spare the time to explain a) assuming the TX is OK which I think it is, why I'd get sparks on connecting the scope probe ground and b) I don't fully understand the half mains voltage to mains earth bit.

Thanks - Andrew

[G6Tanuki]

Look at the input side of the mains-transformer. There's a good chance you will find two capacitors connected in series across the mains, with the centre-point connected to earth.

This is intended for interference suppression, but also acts as a capacitive potential divider and, if you've lost an earth, can easily result in the chassis sitting at something like half-supply-voltage.

G6Tanuki

[Lucien Nunes]

If the chassis is floating it will take up a potential determined by the resistive and stray capacitive leakages to both sides of the mains and earth. If for example the leakage is coming predominantly from the transformer and the interwinding screen has a similar leakage to one end of the primary winding as the core has to the other, then the chassis common will see a voltage divider across the mains composed of just these leakages and take up a voltage somewhere near halfway between them. The same situation will arise if there are similar capacitors from mains to chassis, causing a controlled amount of leakage but appearing high-resistance to the insulation tester.

The potential might be high (hence the sparks) but so is the source impedance, making it behave normally when earthed. The input impedance of a digital meter is higher still, enabling this potential to be measured without it collapsing. Many small double-insulated switched-mode PSUs behave like this due to their suppression capacitors to either side of the mains, delivering a steady earth leakage of around 0.25mA into whatever earthy connection might be made to their isolated output but giving some arbitrary high voltage reading when that connection is only via the megohms of a DMM.

Non-earthed vintage kit that floats up live is less likely to have suppressor caps working normally and more likely to have a fault such as transformer insulation failure. Meggering the primary was a good move.

You sometimes see references to live-chassis TVs etc with the chassis at 'half mains potential'. This is a different situation where the chassis is connected to one side of the non-isolated DC rail and hence electrically common to neither side of the mains. This kind of setup is obviously unsuitable for lab equipment due to the need to make external connections other than the capacitor-isolated aerial input of the typical live chassis TV.

Lucien

Oops - Previous post arrived while I was typing

[Andrewausfa]

Thanks gents, transformers and mains supply are not exactly one of my strong points at this stage of the learning curve which is why I asked.

The set does have two Mica capacitors across the mains. Other than the mains transformer which shows no external signs of distress, these were one of my thoughts. These would be better replaced with modern X2 types I assume?



Thanks for your help _Andrew

[Ed_Dinning]

Hi Andrew, if they are directly across the mains the X types are OK, if their centre point is to chassis they MUST be Y types and a lower value (typically 1000pf or so).

Ed

[McMurdo]

Is there any chance the sig gen was originally earthed and someone's tried to convert it to floating operation in the past? If that's the case, I would consider fitting a 3-core mains lead for safety's sake.

[peteratnith]

Hi Andrew, all good advice but I would fit a 3 core mains lead and earth the chassis as well. If you dont you risk putting a large voltage spike of indeterminate polarity into whatever you are feeding. I cant see any drawbacks to this and it will avoid those jolts you get at present!. Advance E2 I think, remarkable gennies these, I recently replaced the original electrolytic on mine even though it showed no signs of distress. I set up the counter to check the calibration, it was so close I didnt have to touch anything - after 60 years!

Peter

[Andrewausfa]

Thanks for the advice.

Unluckily for me I've plenty of X2 but no Ys. Just ordered some so thanks for the reminder Ed.

As far as I can tell this particular generator has always been without an earth. The two core mains lead looks original with a rubber boot retainer over the turned back sheathing.

Peter, oh so close, it's an E1....but an E1 with a baffling twist. I shall reveal all when it's safely working. Yes, they are good. I have three plus a J1

Andrew

[mrmagnetophon]

I would just earth it, I have some recorders that float at 60 volts AC, and got a shock and could feel the current flowing when I touched a grounded rack.
-Chris

[ted_ntsx]

Originally Posted by G6Tanuki

Look at the input side of the mains-transformer. There's a good chance you will find two capacitors connected in series across the mains, with the centre-point connected to earth.

This is intended for interference suppression, but also acts as a capacitive potential divider and, if you've lost an earth, can easily result in the chassis sitting at something like half-supply-voltage.
G6Tanuki

Many years ago I had a big old Pye PMR transmitter (converted for amateur band use). It was all metal, chassis and case. Each pin of the mains socket had a standard small silver mica capacitor to chassis. The type of cap you find in oscillator circuits!

The cap from the live pin was S/C so the whole case was at full mains potential. I discovered this violently when I connected an earthed lead to it. Fortunately, I survived the incident and learnt something that day...

Ted.