DISCRIMINATION BETWEEN RCDs

[Piratepete]

Morning Guys

I know that if a 100mA RCD is followed by a 30mA RCD, the 100mA needs to be S type.

If a 300mA RCD is followed by a 30mA RCD (Farm installation) does the 300mA need to be S Type?

Cheers

Pete

 

[SPARTYKUS]

Yes. The mA value is irrelevant for discrimination. Think about what happens when there is a fault, follow its path, think about the speed of operation of devices, and the penny will drop

 

[Piratepete]

Yes. The mA value is irrelevant for discrimination. Think about what happens when there is a fault, follow its path, think about the speed of operation of devices, and the penny will drop

Cheers
Just looking for confirmation because of the extortionate price difference between standard and S type.
Pete

 

[SPARTYKUS]

Yes they can be pricey!
But think, if you fitted a 300mA then a 30mA 100 metres away further along, and a fault occurs, which one will trip?

(Trick question)

 

[Chr!s]

Well, If we Have a leakage current of 300 mA, downstream of the 30 mA RCD, so we have the 300 mA TD first, then the 100 mA TD and finally the 30 mA.

1, 300 mA TD rcd
2, 100 mA TD rcd
3, 30 mA rcd

Which Rcd will trip?

Regards Chris

 

[Engineer54]

Morning Guys

I know that if a 100mA RCD is followed by a 30mA RCD, the 100mA needs to be S type.

If a 300mA RCD is followed by a 30mA RCD (Farm installation) does the 300mA need to be S Type?

Cheers

Pete

You often see statements like, ''The upstream RCD device needs to be a minimum of 3X the mA rating, that of the downstream RCD device to assure discrimination''!! If you follow this rule, your going to be sorely disappointed, because that statement is just pure folly and doesn't work in the real world!! It won't work with 10X the downstream mA rating either, and have personally known 30mA and 500mA devices tripping out together. The ONLY way to ''ENSURE'' discrimination between upstream and downstream RCD devices during a fault situation is by Time Delay on the upstream device...

In industrial and larger commercial installations, the use of adjustable RCD modules to give even greater control /closer protection on intensive RCD protected installation areas.... If you think S type RCD's are expensive check those babies out...lol!!! Both mA and time delay ....10mA to 1A rating settings and time delay 0 to 60 sec

 

[SPARTYKUS]

He's right. You can get a 30mA time delayed RCD if you really want ( more than 40 nickah too!!)