rcd

[Engineer54]

There is an interesting article in jan PE mag on this...and it argues that a 100ma S type will fall foul of reg 531.2.9 re- ensuring downstream devices tripping before upstream in the event of a fault. The conclusion was a minimum of 300ma S type would be required for final circuits with 30ma protection.

100mA or 300mA it won't make too much of a difference on a TT system. So what was the argument put forward for a 300mA S type over a 100mA S type?? Level of fault?? I would think most actual fault levels, rather than an increasing combined leakage, will exceed 300mA as well... lol!!

 

[MDJ]

Ok why a s type 100ma

why not?

The time delay will allow the circuit protected by the RCBO to trip first in a fault condition, it is also there as a back up should the RCBO fail, there are also some sparks (uhum, cough) who think a double pole main switch is the best way and individual rcbos after (cough) but there will be trouble suggesting it, ahem cough

 

[Deleted member 26818]

No 1 can answer it can they

Yes they can.
With a TT installation, overload protection can be provided as with any other installation by a fuse or MCB.
However due to the High Zs in most TT installations, earth fault protection cannot be provided by such devices.
Therfore the options are to use an RCD, electrical separation, earth free location or double insulation.
As most of those options are not practical, or insome cases allowed in domestic installations, RCDs are used.

There is a requirement to divide an installation into separate circuits, so as to amongst other things minimise iconvenience in the event of a fault.
So we use a time delayed RCD which used to set so that it operated within 5secs, but now has to be set so that it operates within 1 sec.
100mA is used, to avoid nuisance tripping from combined earth leakage from multiple circuits.
10mA earth leakage on 3 circuits will not cause a 100mA RCD to operate, but would cause a 30mA RCD to operate.
Circuits that require 30mA RCD protection are then provided with such protection, and the 30mA earth leakage required to cause such devices to oper ate will not also cause the 100mA RCD to operate.
The reason why the 100mA RCD is time delayed, is in case of an earth fault causing a current greater than 100mA.
An earth fault of 100mA would cause wither the 30mA RCD or the 100mA RCD to operate, and ins ome cases both.
By delaying the 100mA RCD, the 30mA RCD will operate before the 100mA RCD has a chance.
Obviously an earth fault between the 100mA and the 30mA RCDs or on a non 30mA RCD protected circuit, will cause the 100mA RCD to operate.

 

[toptrics]

So basically the 100ma protects not only final circuits if the 30ma fails it protects the distribution side ie fault occuring on meter tails and none rcd protected circuits.

 

[Deleted member 26818]

Yes.....