MCCB with RCD settings

[Bradley6969]

MCCB with RCD bolt on, anyone know how to work these settings out? Ignore settings now but this is upfront RCD TT system so I want to top one to be 0.3 for 300mA but unsure on bottom?

 

[Richard Burns]

The RCD protection for the final circuits running to the sockets is this RCD unit.
The maximum time allowed for disconnection of a final circuit, rated 63A or less, containing socket outlets, on a TT system, is 0.2s.
The sockets themselves have local 30mA RCD protection.
You need to ensure that if there is a fault on a socket that the local 30mA RCD trips before the main RCD unit you pictured, to avoid disconnecting other circuits unnecessarily.
The 30mA RCDs will have no pre set time delay so they must disconnect within 40ms (0.04s) when there is a significant fault current (in testing terms when the current is 5*Iδn). (or 300ms at Iδn)
Therefore in order to provide at least partial discrimination to ensure the individual sockets disconnect first you have to set the time delay for the RCD unit to above 0.04s and below 0.2s.
To ensure, according to the maximum limits in the regulations, that each socket always went first you would have to set the time delay above 0.3s but this would not be allowed by the maximum disconnection time limit of 0.2s. Most RCDs would practically disconnect far faster than 300ms at Iδn so discrimination would be achieved in practice at values lower than 300ms, perhaps 100ms might be a good compromise?

 

[Deleted member 26818]

BS7671 appears to have dropped the requirement for 30mA RCDs to operate within 40ms.

 

[davesparks]

That is commonly called a vigi module. You will notice when set for 30ma there is a line down indicating to set to no delay so additional rcd protection is provided.

Strictly speaking vigi units are a Schneider (previously Merlin Gerin) product range.

 

[Ffoeguk]

If it’s a distribution circuit fed from the 300mA then disconnection time is 1 sec
Set it up then test for compliance with your red tester.
30mA rcd for sockets are correct for final circuits fo protection of persons or livestock.
300mA used as fire protection for earth fault on the distribution cable.

 

[loz2754]

BS7671 appears to have dropped the requirement for 30mA RCDs to operate within 40ms.

The requirement is still there, according to BBB 643.8 and OSG 11.5

I think some confusion may have arisen due to the dropping of one of the columns on the test sheet. There is now only one column for trip time. If additional protection is required for a particular circuit, you would record the value measured at 5 X the rated operating current. Otherwise you would record the value measured at 1 X the rated operating current.

 

[Deleted member 26818]

The confusion was caused by the change to 415.1.1.

 

[loz2754]

The confusion was caused by the change to 415.1.1.

Agreed.

It makes little sense to remove the 40ms reference from one reg but still require it in another section.