MEM RCBO Pods not tripping whilst under test (a new one on me).

[Dunc]

I know i've mentioned the word 'test' in the thread title however this is a piece of information that might be of interest to all in the trade considering the requirements for RCD protection in BS7671 2008 hence why it's in the 'General Electrical Forum'.

Anywho, to set the scene, been carrying out some work in a property with a Memera 2000 consumer unit. The work involved additions to lighting circuits in the bathrooms so I decided to fit an RCBO pod to the existing MCB to provide me with 30mA protection.

Fitted the pod which was a straight forward enough job and checked the test button worked before carrying out further RCD tests with my Megger MFT. Unfortunately, at this point things started to go wrong. The breaker refused to trip when applying tests to the terminal screws at the CU which i do with the circuit disconnected so as to avoid damaging equipment which may not like being switched on and off in rapid succession. I even carried out a 100mA ramp test but still it stayed on.

Assuming i had a faulty unit i disappeared off to the wholesalers to get a replacement pod and an hour later returned to site to finish off. Got the new pod fitted, operated it using the test button but again when applying test currents it refused to trip! Now, being worried that it's my test meter on the blink i try the same tests on the adjoining RCD which goes without a hitch. Assuming i must be missing something i try a Wylex RCBO bodged in place to prove a point. This RCBO trips fine so now i decide to give MEM technical a call to see if i'm being stupid. Run through what i've done with the guy on the phone and he tells me where i'm going wrong!

Anyway, the guy at MEM tells me the problem is when testing MEM RCBOs at the device's terminals the test current goes round the outside of the CTs within as it is being applied too close to the unit. His solution was to connect a few inches of wire to each terminal and clip on to these wires to carry out the test. I was a little sceptical when he said this having never had problems before but duly did what i was told. Sure enough it tripped like a dream giving me readings of 38ms @ 30mA and 28ms @ 150mA!

Talk about a ball ache but atleast it's all sorted and i can now sit down, sign off and invoice the job. Thought i would share this one with you guys as i'm sure you'll come across similar problems at one time or another. Also, has anyone had this happen to them on MEM or other products. Would be interested to know.

 

[davethsparky]

The pods were a good idea at the time and did work out to be cost effective, especially when adding RCD protection to an existing circuit. They don't make them for the new memshield 3 MCB's as far as i'm aware.

The point about testing them is obvious from looking at them though isn't it? The RCD pod clips on to the top of the MCB and the live passess through it to go into the original MCB terminal, doesn't take a genius to realise the sensing element has got to be in the pod and so testing at the live terminal will be testing before the thing you are trying to test. Its got as much chance of working as testing at the incoming side of any RCD would!

 

[superspark]

These units are NOT the pods that clip on, these are RCBO's, and testing from the terminals, the same as other RCBO's is the problem, with the download I posted, there was a picture which didnt post.

54, I had this problem, thursday night, so I hadnt read the original post, I contacted Meggar in the morning, and they advised me of the situation, and sent me the information, which they had obtained from MEM, but apparently, they where a bit cagey on admitting anything

 

[Darkwood]

I use Schneider gear; end of! :conehead:

 

[superspark]

At Last I have down loaded the picture, it does help to explain better