Isolation Tx, Bonding of Metal Parts

[Mr Malcontent]

Hi all, quick question, I have come across a 230V control circuit supplied from an Isolation Transformer. The controls are located within a metal clad enclosure near to local socket outlets and other accessories.

I have measured continuity between the metal casing of the controls enclosure and the main earthing system on the primary side and measured 10ohms?

Am I correct in thinking that there should be no continuity between associated metal parts on the secondary side of the install and the main earthing system? There should be no Earth bonding between the controls enclosure and the main earthing system if I am understanding the principles correctly?

There is no Earth point at the transformer.
Cheers all.

 

[David Prosser]

As long as there is no continuity between the secondary coil and the metal work then there’s no problem.

 

[Mr Malcontent]

As long as there is no continuity between the secondary coil and the metal work then there’s no problem.

There is no visible N-E bond at the transformer. I just wasn’t sure if the 10ohms was acceptable as I was just expecting to measure a very high impedance to be honest. The engineer on site is saying they have only bonded all metal parts on the secondary side I.e controls enclosures, etc but not taken it back to the primary earthing system so a little surprised at the continuity.

 

[Lucien Nunes]

You can't stop metal equipment coming into contact with earth one way or another unless you mount it on insulators, so it is not normally a requirement that it is isolated. Boxes and conduits screwed to machines or building framework, the odd screw that goes through concealed metal studwork, even conduit buried in concrete can all bring a relatively low resistance earth to a system that has not been deliberately earthed. I would normally earth such a system.