time delay rcds

[Taylortwocities]

Thanks Engineer 54 , i had never heard of the x3 recommendation .

That's because for RCDs it doesn't happen. As Eng 54 has said.
I rather think the x3 refers to discrimination of fuses, mcbs etc.

Lets say you have a 300mA RCD main switch(non time delayed) and sockets protected by a 30mA RCD.
Which RCD will trip first if there's a big earth fault on one of the sockets of more than 300mA? Its the RCD with the faster (time) trip characteristic or it may depend on where the AC sine wave is at. It could really be either, or - sometimes - both of them.

An S-type is the only way to ensure discrimination of RCDs.

 

[JD6400]

Thanks Taylortwocites , that would generally be the case as you say . It was the 3x rule of thumb that i have not heard of before , well for RCD's anyway .

 

[Engineer54]

That's because for RCDs it doesn't happen. As Eng 54 has said.
I rather think the x3 refers to discrimination of fuses, mcbs etc.

Lets say you have a 300mA RCD main switch(non time delayed) and sockets protected by a 30mA RCD.
Which RCD will trip first if there's a big earth fault on one of the sockets of more than 300mA? Its the RCD with the faster (time) trip characteristic or it may depend on where the AC sine wave is at. It could really be either, or - sometimes - both of them.

An S-type is the only way to ensure discrimination of RCDs.

No, this was a stated quite clearly in a few of the manufactures catalogs in the past, Merlin Gerin, Siemens being two of them.

Discrimination using this method (without a time delay) say a 100mA upstream of a 30mA, only works with increasing natural leakages, any fault on that downstream circuit that produces a fault leakage current exceeding 100mA, ....and both RCD's are likely to trip simultaneously.

As you say, all RCD's/RCBO's have a pretty wide ranging tripping speed, which can also affect discrimination between series connected RCD devices. But the only way to ''Ensure'' discrimination in all normal circumstances, is by means of an S type device being used upstream...

 

[infinity]

Also there would be no need to use a 100mA rcd main switch if all circuits had 30mA Rcbo's would there?

 

[Engineer54]

Also there would be no need to use a 100mA rcd main switch if all circuits had 30mA Rcbo's would there?

Your thinking too small here. There are far more different distribution layouts than a domestic CU, be they dual RCD or all RCBO layout!!

 

[Taylortwocities]

Also there would be no need to use a 100mA rcd main switch if all circuits had 30mA Rcbo's would there?

But you are correct, for a domestic TT all RCBO there is no need for additional 100mA RCD incomer.
Just a normal DP main switch.

 

[Engineer54]

But you are correct, for a domestic TT all RCBO there is no need for additional 100mA RCD incomer.
Just a normal DP main switch.

That depends, what are you going to do for say a supply going out to an outbuilding or garage with a mini consumer unit protected by it's own 30mA RCD device??

 

[Tonkatoy27]

Time delay RCD doesn't have to cost the earth. I hate to say it but Denmans Control Gear Direct 100mA S type is dirt cheap.

 

[alarm man]

But you are correct, for a domestic TT all RCBO there is no need for additional 100mA RCD incomer.
Just a normal DP main switch.

i thought you needed the 100ma as main switch if the cu was metal.that seems the norm on the ones ive seen

 

[Engineer54]

i thought you needed the 100ma as main switch if the cu was metal.that seems the norm on the ones ive seen

I think your right there, but it would still have to be an S type, if you wanted to maintain discrimination!!

 

[JD6400]

i thought you needed the 100ma as main switch if the cu was metal.that seems the norm on the ones ive seen

I may be wrong , but if using a metal cu it will need a insulated shield around the incoming part / section of the cu to give extra protection to the tails before the rcd .
This may not be applicable now days , but it seemed to be standard practise on nearly all of the older TT boards i have come across ?
I hope you do not mind me asking Engineer54 ,or anyone else to that matter .
Was this the case in the past or have i been misguided to believe so ?

 

[alarm man]

I think your right there, but it would still have to be an S type, if you wanted to maintain discrimination!!

sorry e54 forgot to add s type

 

[alarm man]

I may be wrong , but if using a metal cu it will need a insulated shield around the incoming part / section of the cu to give extra protection to the tails before the rcd .
This may not be applicable now days , but it seemed to be standard practise on nearly all of the older TT boards i have come across ?
I hope you do not mind me asking Engineer54 ,or anyone else to that matter .
Was this the case in the past or have i been misguided to believe so ?

i think the 100ma is to protect the tails on metal clad boards,tho it may be omitted now if using insulated seals as you say but dont quote me on that

unlike the pic shown in the onsite guide,the 100ma was normally a stand alone unit placed between the meter and the cu if metal clad,the main switch was a standard main switch in the cu.

 

[JD6400]

Sorry Alarm man , got the wrong end of the stick with your post .
I took it that you meant the main switch was in the cu and not a stand alone !:49:

 

[alarm man]

Sorry Alarm man , got the wrong end of the stick with your post .
I took it that you meant the main switch was in the cu and not a stand alone !:49:

sorry thats why i edited mate,did sound confusing..

to edit again,the osg pic of tt is confusing too,just states additional protection required in using metal clad cu,so as you said insulated glanding into board,(ive read it somewhere) or swap the 100ma in the cu with a main switch and fit a s/a 100ma between the meter and cu