Problems with more than one rcd

[John5252]

Hi all, just wondering if anyone with a lot of experience had any thoughts on this;
I've seen on farms that are usually tt earthed the mains come in and through an s type time delay rcd and then sub mains go to different buildings which are protected by standard 30ma rcds. Sometimes faults in individual buildings will only trip the individual rcd but sometimes the main rcd will trip, losing power to all buildings.
would providing an earth spike for each individual building rather than just relying on one back at the mains improve things?
or is it just one of those things and using an s type rcd is all that can be done?
cheers

 

[Octopus]

You shouldn't have 2 x RCD's of the same values in series, which is what your post suggests. Adding a spike at each building will not change anything.

 

[Zebra Stripe]

As above, lower Ze wont make a difference, have you tested the time delayed RCD? cos if its faulty and not holding, you have whats described above 2No RCDs of same value and who knows which will disconnect first

 

[Richard Burns]

Assuming you are talking about the time delayed RCDs being 100mA or higher then the only reason a time delayed RCD would trip before a non time delayed RCD is because of a faulty RCD or a simultaneous double fault.
There is the possibility of a time delayed RCD tripping as well as a single pole 30mA rcd and this would be on a neutral earth fault. To overcome this then you would need to use double pole RCDs.
Using multiple earthing systems across an installation would require careful though and planning but would not prevent the above scenarios. Because an RCD will trip on a Line Neutral imbalance, if the installation has a common supply, irrespective of the means of earthing, then any Line Neutral imbalance will be seen at all points in the installation upstream of the RCD.

 

[Zebra Stripe]

wow, Im impressed, you know your stuff:icon7:

 

[John5252]

It's a 100ma at the mains and 30ma in separate buildings. Will test and see at what times they trip at. Cheers guys

 

[GLENNSPARK]

It's a 100ma at the mains and 30ma in separate buildings. Will test and see at what times they trip at. Cheers guys

100mA....or 100mA S type?

 

[John5252]

100ma s type

 

[GLENNSPARK]

well you should be getting discrimination between the 30mA and the 100mA s type..in favour of the s type..(thats the whole point of em)...

go get a leakage clamp on the origin...see how much leakage that s type is being subjected to...

 

[Croney]

May also be worth Ramp testing the s type just to be sure.....