Residual Voltage Devices

[jpflying]

Can anybody tell me how a residual voltage device operates. I know how a residual current device (RCD) operators, but not a residual voltage device.

Cheers

 

[ScottyUK]

Voltage-operated is a bit of a misnomer: the main earthing conductor passes through the device's sense coil. When current flows via the earth conductor the secondary winding of the sense coil develops a voltage which, if large enough, trips the relay. They are dangerous because it is possible for a secondary route to earth to exist via an earthed water main, an appliance in contact with the mass of earth, etc, which either desensitises or bypasses the device.


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If we learn from our mistakes I'm getting a great education!

 

[jpflying]

ScottyUK

Thanks for the reply.

We are currently reviewing the earthing requirements for generators in surface coal mines (Queensland, Australia) and a discussion paper has come to us from our counterparts from another Australian State where they talk about single phase generators with voltage sensing relays. None of the members of our group have had experience with them as they appear to be rarely used. The connection diagram in the paper shows the earthing system is of the isolated type ie earth is not connected to the neutral conductor.

 

[ScottyUK]

Yeah, they were popular in the UK in the 50's and 60's but many have been replaced by a current-based RCD. There are still a lot out there in older properties. I'm pretty sure voltage-operated devices are prohibited under the current UK regs, either explicitly (in which case I can't find the clause) or implicitly by the performance requirements for an RCD which are specified.

I would use a standard RCD of the type you're familiar with - you can selectively apply them to individual circuits, you can achieve at least one level of coordination by time if necessary, and there isn't an inherent way of inadvertently defeating the protection.


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If we learn from our mistakes I'm getting a great education!