My volt meter melted when measuring btween exposed metalwork

[Alarm]

Yes Pls, pics always good.

 

[securespark]

I would be. Always interesting to see inside the big stuff.

In that case, would you like to see my never mind, I'll get my coat.

 

[Monkeh]

I would be. Always interesting to see inside the big stuff.

In that case, would you like to see my never mind, I'll get my coat.

And stay out!

 

[martinxxxxxx]

There may also need to be an earth rod under the machine to disipate stray RF energy. Has that failed ?

The chap doing the repair thinks that this would be a good idea but it would in volve drilling through the factory floor. Do you think this is worth it as there is a doorway within a meter of the machine that we could put the spike just outside of?
What is the max impedance of the spike for this do you think. I was going to drive down until I achieved sub 20ohms?
Also I have read that RF systems may require a more specialised earthing system. Such as long copper threads shallow buried radiating from the point of transmission (obviously unachievable here) but your opinion would be welcome.
I intend to treat this machine as a TT and not bond it to any other earths or metalwork but I worry that this approach could lead to PD between the machine and portable class 1 tools brought near it. On the other hand if I bond it to the local earth (10mm cable at thewall) we may end up with RF all over the place. I hear a rumour that this may have been removed in the first place due to interference to VDU screens in the offices upstairs. Obviously a lot depends on what value of impedance I can achieve on the spike?

Any thoughts

 

[bernardgreen]

There may also need to be an earth rod under the machine to disipate stray RF energy. Has that failed ?

The chap doing the repair thinks that this would be a good idea but it would in volve drilling through the factory floor. Do you think this is worth it as there is a doorway within a meter of the machine that we could put the spike just outside of?

When dealing with RF energy all bends in a cables introduce inductance into the cable and thus increase the impedance of the cable to tens if not hundreds of ohms.

What is the max impedance of the spike for this do you think. I was going to drive down until I achieved sub 20ohms?

Pervously for disipating RF energy into the ground the DC resistance of the rod to soil interface can be very high. In which case the RF energy can still disipateinto the ground by capacitive coupling from rod to soil.

Also I have read that RF systems may require a more specialised earthing system. Such as long copper threads shallow buried radiating from the point of transmission (obviously unachievable here) but your opinion would be welcome.

That is definately the case when the site is a transmitter site and RF energy is being intentional sent out from the site. In the case of diathermic heating equipment the RF energy should ( in theory ) be confined to the heating head and the material being heated. In practise there will always be leakage from from the head and the material and also from the RF cables between the RF generator and the head. Good machine design will minimise the leakage. But that type of design is more a black art than theory.

I hear a rumour that this may have been removed in the first place due to interference to VDU screens in the offices upstairs.

That is a strong possibility.

I intend to treat this machine as a TT and not bond it to any other earths or metalwork but I worry that this approach could lead to PD between the machine and portable class 1 tools brought near it.
Any thoughts

A valid concern, one option would be to install voltage detection between the chassis of the machine and the earth used for sockets for portable tools. In the event of a PD between them then the supply to the machine is cut. Either manually in response to warning lamps (and sirens maybe) or automatically if that will not create hazard or harm to the machine. The voltage sensor would need to be RF immune which is reasonable easy to achieve using RF chokes on the sense lead from the machine's chassis.

If you do end up drilling through the floor for a local ground rod make sure the factory plans of under floor services are accurate. An engineer doing similar in France hit oil, hydraulic oil from a hydraulic main running in a tunnel under the floor.

 

[barnzi]

You could get a good RF earth through one or more capacitors and an electrical earth through a choke. That's exactly how RF PLC systems are coupled to 138kV lines, except the RF goes to earth via a radio modem.

That's how I'd approach the problem as an electronic engineer who builds high power (kilowatt) transmitter.