Type A RCDs Vs Type AC

[tmus1]

I understand why the increased use of type a rcds are recommended however there’s something I don’t understand. If these type a rcds aren’t tripping at 1x on a normal 50hz ac cycle or at x5 then surely they can’t be fit for purpose because they are not detecting a potentially critical fault. In my understanding an MFT simulates a fault at a specific leakage current in order to trip the rcd. Type ac rcds trip without failure most of the time. But carry out exactly the same test under the same conditions on a type a RCD it fails to interrupt the supply. Obviously there’s a type a test on the mft but why is it acceptable that they fail the ac test when the circumstances and requirement for them to trip under fault conditions on an ac circuit hasn’t changed?

 

[davesparks]

Manufacturers specify a maximum earthing resistance for their RCBO's, this may not always be met with TT systems.

For example schneider specify a max earthing resistance of 100 ohms.

 

[westward10]

There was and is a requirement to disconnect all live conductors for a circuit protected by an RCD

And now they have rigged the rejigged the regs to allow single pole RCBOs

Can you point me to it. Are we talking rcd or a circuit protective device such as an rcbo.

 

[Charlie_]

Can you point me to it. Are we talking rcd or a circuit protective device such as an rcbo.

I am talking about RCDs but in doing so that also includes RCBOs and other such devices

 

[westward10]

I am talking about RCDs but in doing so that also includes RCBOs and other such devices

I am talking rcbo, there is no requirement for them to disconnect the neutral on a TT system where their function is only circuit protection.

 

[Charlie_]

Manufacturers specify a maximum earthing resistance for their RCBO's, this may not always be met with TT systems.

For example schneider specify a max earthing resistance of 100 ohms.

Hager used to stipulate no sp RCBOs for tt systems and now it’s all ok..
Why?
Because it suits what they are selling

 

[Deleted member 26818]

There was and is a requirement to disconnect all live conductors for a circuit protected by an RCD

And now they have rigged the rejigged the regs to allow single pole RCBOs

If they have rejigged the regs, they did it back in the 1980s.

 

[davesparks]

Hager used to stipulate no sp RCBOs for tt systems and now it’s all ok..
Why?
Because it suits what they are selling

Or maybe they have changed the design of their RCBO's to be suitable for use on a TT system.

I don't fit Hager so I don't know, do they have functional earth's for their RCBO's or are they the type without?

 

[westward10]

The new compact rcbo has no functional earth.

 

[Charlie_]

For years I have used dp RCBOs on tt systems - MG or Hager and will carry on doing so..
Seems strange that the regs are ok with not disconnecting the neutral on a tt system even though it’s not tied to earth.

 

[westward10]

It's always been that way and wasn't an option with fuses.

 

[Charlie_]

But it should should be on tt systems, still had main earth leakage device at the source

 

[westward10]

Maybe but BS7671 only stipulates it for isolation not circuit protection.

 

[Charlie_]

It stipulates RCDs shall disconnect all live conductors for the protected circuit except for protection against electric shock

 

[Deleted member 26818]

If we need double pole protection in TT installations, then fuses can’t be used.
So we have to condemn all those BS 3036 and BS1361 fuse boxes.
What about the DNO fuse, or plug top fuses?

Wait, what about single pole light switches?

 

[Strima]

Stop picking holes you lot!!!