Type B time delayed rcd

Although a type B will trip in a situation where an A  or AC type may be blinded and not trip, the reverse is  not true.

IF an A type trips, a B type with the same ratings and delay should trip as well.

I'd not bother with trying to get a B type until they can explain and then tell you how to stop the A type tripping.

I tried something similar several years ago when Type A RCDs weren’t do readily available.


One of the RCDs I bought and tried but tripped is still in my van looking for a good home, one day its time will come.


Andy B.

Several years ago when I was working with those RCDs the upfront 100mA Time delayed RCD was tripped by an inverter, but the downstream 30 mA RCD didn’t trip, presumably because it was being blinded, I have had the same again with Bosch direct drive washing machine in a different house.


At that time the best quote for a Type B RCD was £740 plus p&p making it over £750, hence the lack of desire to go straight to fitting a Type B only to possibly have it trip. At least now they are relatively cheap at around £120.


Andy Betteridge

The big issue is why the RCDs that don’t trip don’t, rather than why others do trip, such as why doesn’t a 30 mA RCD trip when the 100 mA RCD upfront of it does ?

The hidden question is should it really have tripped or should it have ignored it ?

DC blinding is one thing, and larger RCDs are harder to blind, as the cores are larger area, but with switching electronics there are other possible factors, and RCD specs are not written in a way that defines 'correct' behaviour for all cases.

Very short duration high amplitude spikes may be perfectly safe - that electrostatic 'nip' you get from combing a cat or running on nylon carpet may be a few kV and tens of amps flow though your finger tip, but it is only there for 10 microseconds and produces no significant heat, and no burns or life threatening muscle twitch - should an RCD respond to non- dangerous events that or not ?


A switching supply or inverter may well introduce just such a glitch every time the internal switching transistors reach a particular point in their cycle  of changing states.  That may or may not mean there is a problem that needs a trip.


Of course if the things were properly filtered then the short spikes of kV and microseconds on the dirty side of the filter would be spread out into milliseconds of a few volts and milliamps on the clean side, so the RCD wold then not see it, but the "value engineering" teams have taken out any filtering that is not mandatory smaller lighter cheaper product is always a competetive advantage. Making it work is someone else's problem.


In terms of 50Hz leakage it is perfectly possible to differentiate between capacitance between L and E,  which is leakage through filters and so  on, and benign, and resistance which could be a human, but no RCD I know of does look at the I-V phase shift to do this, and if it did it probably would not meet any of the product standards.

In the example of the no-trip 30mA and the firing 100mA, if it was  DC flowing then I agree that is very bad, and the 30mA is being blinded.  If it was short duration switching spikes, then it is not clear which if any RCD should have tripped. The problem is, rather like the conducted emmissions question of the LED drivers in this thread , without kit that most folk do not have, you can only really 'try another model' and see if it is any better, which can be very expensive and unsatisfactory, a bit like proposing to find the deepest crater on the moon by walking round with a bag over your head...

Several years ago when I was working with those RCDs the upfront 100mA Time delayed RCD was tripped by an inverter, but the downstream 30 mA RCD didn’t trip, presumably because it was being blinded, I have had the same again with Bosch direct drive washing machine in a different house.

Possibly not blinding as such, but maybe just lack of sensitivity to some waveforms. If the "leakage" was pulsed d.c. say - an A-type 100mA should trip, but a downstream AC-type 30mA is permitted to ignore it. Even if both were AC types it might just happen that one still has some sensitivity to pulsed d.c.


   - Andy.

Baldyhugh:

Hi

I researched the spec of the drive which says that I need a type B time delayed rcd. 

 

Stop messing about and bounce your problem off of the supplier.


Jaymack

I agree. Get the manufacturer to scratch his head on your behalf.

BS7671 states that manufacturers instructions must be followed - everything else follows on from that.

I had similar issue with a ground source heat pump manufacturer - given that as early as 12 months ago I was getting quotes between £450+VAT and £750+VAT for Type B/F RCDs, there was no way I was committing big money on a shot in the dark.

So I nailed the manufacturer down to specify the precise item he required and moved on from there.

I ended up with a Chint 80A DP Type B for £119 inc VAT.

As for the tripping - get them in to check their inverter product - there must be some kind of warranty?