Hi,

I need some advice on the mains wiring of the subject device. I have just removed the main plug to fit a new grommet to the case and a new plug. The cord is the shielded type but the shield was not connected to the plug. I didn't take any note of the connections before removing the old plug and preparing to connect the new plug. I connected the wires correctly at the plug but before plugging in, decided to make sure everything was ok,,,,,,,,,,,surprisingly, the earth did not appear connected so I removed it from the case and shock horror, found the grey connected to the variable voltage selector and the red clipped. Luckily, the red was not near the chassis and had no stray wires around it. Found the wires I removed from the old plug and saw the green was used for active and the black for neutral. The braided shield was connected internally to the chassis but not at the plug and there was no earth connection.

Before I fit the mains cord and plug I would like to know whether or not the earth is connected to the chassis and plug and where the shied connects ie the plug or the chassis or both. Below is an extract from the manual https://www.antiqueradios.com/forums/viewtopic.php?f=8&t=234188 which to me suggests the braid only is connected to the earth pin at the plug and not the chassis:

The earth terminal on the left side of the device must be properly grounded before the device is connected to the mains.
When setting up the measuring bridge, care was taken that there were no alternating or magnetic alternating fields in the vicinity of the center bus (Bu2), as this could result in an incorrect indication or an unclear minimum when measuring high impedances.
To prevent malfunctions via the power cord, this is shielded while the shield is connected to the earth terminal.

I have another instrument the same so decided to have a look at the plug end but neither the shield or earth were connected. What I found was the grey was connected to active and the red clipped. Whilst the instrument works fine it is disturbing to find the green used as active. Any advice on how to wire correctly would be very much appreciated.

As a matter of electrical safety, the metal chassis must be earthed. These days that means via a crimped lug on the earth wire and connected to a bolt with a star washer. Also, a grommet by itself is no longer considered adequate. These days a cable clamp or cable gland is required.

Are you saying that the mains cable is shielded? If that's the case then I would expect the shield to be connected to the aforementioned chassis earth point end only.

When you wire the plug, ensure that the active wire (red or brown) is the one that goes through the on/off switch if there is one.

As regards interference, the manual says:

3. Buses Bu1, Bu2 and Bu3 should not normally be grounded; the only place where the circuit is normally connected to ground is the terminal on the left side of the device. If impedances are to be measured that are grounded on one side, this side must be connected to one of the buses Bu1 or Bu3. In order to obtain correct measurement results, the measuring bridge must be set up in isolation (for example on a glass plate). The circuit is then only earthed via Bu1 or Bu3.

The bridge uses the 50 cycle, (or Hertz), as the source and as you are trying to detect the minimum output when the bridge is balanced, it is essential to minimise any interference from hum or earth loop currents. These stray signals will mask the null when balanced leading to uncertainty of the correct balance point. The seemingly complicated earthing procedure is limit this interference.

It is for this reason that a different frequency source is suggested for some measurements, where the possibility of interference is greater.

Harold

GTC

Are you saying that the mains cable is shielded? If that's the case then I would expect the shield to be connected to the aforementioned chassis earth point end only.

The mains cable is three core (grey, red, black) with a braided shield and a black pvc outer.The shield was connected to a lug and bolted to the chassis but not the plug. If I understand correctly from what you said, it should not be connected to the earth pin at the plug but an earth lead is required from the chassis to the plug. I was a bit thrown by the use of grey/black for the active/neutral (no on/off switch) and the red snipped at both ends but as two instruments were wired like this and no earth is shown in the wiring diagram, I assume it was normal for the times in Holland. Anyway, if an earth is required, I will use the red/black for active/neutral and the grey for earth and fit a cable gland on the case.

The shield was connected to a lug and bolted to the chassis but not the plug.

This is intentional to prevent a current flowing in the braid and causing interference. It is the earth loop current I referred to in my earlier post.

Harold

As Gandhn has said above.

In order to prevent ground loops, such earthing systems need to be star-wired. That is, a common earthing point rather than multiple points.

Ground is provided via the 3-pin plug from the power socket to the chassis common point. From that single common point the ground 'effect' is extended to the cable's shielding.

Thank you all for your responses. I will now wire it correctly with a ground from chassis to plug.