100mA Type S RCD trips, 30mA RCD doesn't

[ccrowe]

A friend's cooker element blew and tripped their main 100mA Type S RCD (it's a TT system) but the 30mA before it didn't trip.

I would have expected the 30mA to go first thus saving the 100mA from bothering with it. Am I missing something here?

 

[Spark123]

The cooker probably isn't on the 30mA side of the consumer unit.

 

[ccrowe]

The cooker probably isn't on the 30mA side of the consumer unit.

It's an old installation. The RCDs are separate units outside rewireable fuse boards. Incomer - meter - 100mA - 2 x 30mA - fuse board. Absolutely everything is under the pair of RCDs.

 

[Spark123]

The next thing to check would be the trip times of the RCDs with an RCD tester.

 

[GaryMo]

Incomer - meter - 100mA - 2 x 30mA - fuse board. Absolutely everything is under the pair of RCDs.

2 x 30mA ?

It sounds like:

Incomer > 100mA Type S RCD > 30mA RCD > 30mA RCD > Consumer Unit.

Is this correct? Is there really 2 x 30mA RCDs?
In ant case, what's the point of a 100mA Type S RCD if there's a 30mA RCD in series just after it?

 

[holmslaw]

..

 

[Spark123]

I see where garymo is coming from, what is the point in forking out for a 100mA S device if all it is doing is feeding a pair of 30mA devices. Why not just feed the 30mA devices direct, of course assuming they are all in plastic enclosures?

 

[GaryMo]

Exactly.

I still read it as 3 RCD's in series which seems very strange

 

[ccrowe]

As per original post, it's a TT system. The 100mA Type S feeds two consumer units, each protected by its own 30mA.

It's quite a complicated setup. The building is a farm house that has been split into two separate dwellings (parents living one side, offspring the other side). The supply comes in to one side of the house, feeds the 100mA RCD and is then split between the two "houses" each on their own 30mA protected rewireable fuse board. The cooker on the parent's side blew and took out the two "houses" because the 100mA tripped and not the local 30mA.

By coincidence I tested the RCDs on my Megger 1553 a couple of weeks ago and all were well within limits (as was EFLI). As I said earlier, it's an old installation and one of the 30mA RCDs (on the side that isn't currently causing me problems) wouldn't trip initially. I needed to press the Test button and reset it a few times to loosen things up. After that all was fine.

I'm going back tomorrow and will retest the RCDs on the Megger. I'm guessing that it's all getting towards end-of-life and needs replacing. That isn't going to happen as cost is a massive issue. I sense one of those doctor-patient type talks coming up where I explain the need for a change and they say yes then do nothing as they can't afford to.

 

[plugwash]

My suspicion would be that at some current the type S is tripping too quickly and/or the ordinary RCD too slowly such that the type S trips first.

RCDs are fickle things unfortunately. It kinda goes with the territory of detecting imbalances of milliamps while withstanding imbalances of potentially kiloamps.