Problem with MCB change

[leeturn]

In my cellar I have a doube socket connected to a type b 16A MCB. When using a 110v transformer it tends to trip the MCB when first plugging the transformer in.
The suggestion was to change the B curve MCB to a C curve MCB.
The CU is a Wylex NH which is populated with NSB type MCBs. Looking on the web it appears that these have now been replaced with the NHX range which is backwards compatible.
This morning I changed the MCB and the cellar sockets were working. However the sockets connected to the 32A MCB were not working. Couldn't work this out as this MCB hadn't been touched so in the end removed the new type C MCB and the sockets started working. Plugged in the old NSB B type MCB and the sockets still work. Plug in the new MCB in a different position and the 32A doesn't work.
I'll get a qualified electrician to come and have a look this week as it does seem a bit strange.

 

[flyingsparks]

There's no reason your new c typecb will affect another circuit. All I can think is there may be a loose connection on the other mcb or neutral which is getting disturbed. Also check the bus bar is correctly clamped into the mcb and not been missed and just touching, causing a intermittent fault.

 

[AndyPRK]

Yeah take a close look at the bus bar, probably not connecting correctly at the 32A.

I'm not convinced NHX are a direct replacement (because of bus bar connections).

Get a correct one off ebay.

 

[leeturn]

Thanks for the reply. The strange thing is that I installed the MCB into a spare slot and connected it up. Switched on and the new MCB was working plus all of the others except for the 32A. Checked the connections and all looked ok but it still wouldn't work. So I removed the new MCB and without touching anything else the 32A then worked. It doesn't make sense and as you say there must be something that is not 100% with the 32A connection.
My nephew is a qualified electrician so I'll get him up this week to have a look.
On another note it appears that 110V site transformers can trip MCBs so is there something that can be used to stop the initial surge or a different type of transformer?

 

[333rocky333]

not sure about the wylex, but a common problem is the bus bar not in the terminal cage, so although the screws tight its not actually connecting.
I have seen this done by alleged qualified electricians

Just moving the bar slightly when you fitted the new one may cause intermittent contact

I presume you checked the earth loop impedance before deciding to change a "B" to a "C" type

 

[JohnW2]

not sure about the wylex, but a common problem is the bus bar not in the terminal cage, so although the screws tight its not actually connecting. I have seen this done by alleged qualified electricians

I can confirm (don't ask how/why ) that doing that is remarkably easy with a Wylex NH board and MCBs! I presume it's most likely to occur when the position of the CU is such that one doesn't have an ideal view of what is going on when inserting the bus bar (and hence may be partially doing it 'by feel').

Kind Regards, John

 

[streetlighter]

Ive had the same problem with my 3.3kva site transformer tripping a B16 MCB as soon as the transformer is plugged in sometimes taking the RCD aswell.
If the transformer is left plugged in the MCB can be reset then works fine same with the RCD !
However it doesnt ever happen with my small 750va transformer.

 

[RF Lighting]

It's called inrush current. When a transformer is first energised, a transient current up to 10 to 50 times larger than the rated transformer current can flow for several cycles.