single/double MCB

[johnnytightlips]

Good afternoon!

A quick question if I may, I am no expert what so ever, been a long time since I worked with a voltage higher than 5v DC

I have mains supply to a DIN rail device which I want to protect with a double pole MCB, however for my current rating requirements (very low) only single or three pole devices are available.

The electrician who has fitted this for me has used two single pole MCBs side by side, for L and N and has then simply placed a clip across the two switches insuring me that 'one will trip the other' etc.

As I say, I am a software eng so please no laughing at my lack of knowledge heh, but is this acceptable practice? Is the trip rating now not double what I want it to be using two MCBs?

Is it not possible or preferable to use just two poles of a 3 pole unit?

Thanks guys,

John

 

[ricicle]

The trip rating will not have doubled by doing this. This is standard practice with certain makes of MCB to acheive double pole devices.

Btw - I have obtained 1 and 2 amp DP MCBs in the past !

 

[Iggifer]

http://uk.rs-online.com/web/p/mcbs/7467696/

1a DP MCB, type C though

 

[ban-all-sheds]

Assuming it's a single-phase circuit, i.e. you don't need to monitor the current on both poles, a breaker with an auxiliary switch would have been OK.

But it's done now.

 

[securespark]

MCB on the neutral??

 

[ban-all-sheds]

2-pole

 

[johnnytightlips]

Morning gents, cheers for the replies!

Don't know why I thought the current rating would be affected... my brain obviously wasn't working!

Ban-all-sheds do you mean something like this - http://uk.rs-online.com/web/p/mcbs/3238983/ (just as an example, don't worry I won't run out and buy one heh).

cheers,

John

 

[Spark123]

Putting a bar across the two MCBs will not make it a double pole MCB, they need to be internally linked. It will switch off both poles when operated manually but not necessarily on overcurrent - you'll probably find that if one operates both the switches will remain up even though one has tripped internally.

 

[JohnW2]

Putting a bar across the two MCBs will not make it a double pole MCB, they need to be internally linked. It will switch off both poles when operated manually but not necessarily on overcurrent - you'll probably find that if one operates both the switches will remain up even though one has tripped internally.

That's what I thought, which is why I was a little surprised when ricicle wrote:

This is standard practice with certain makes of MCB to acheive double pole devices.

However, I was not certain enough of my facts to comment at the time.

Kind Regards, John

 

[ricicle]

I am certain it was Merlin who used to do it. Will try and source some info....

 

[JohnW2]

I am certain it was Merlin who used to do it. Will try and source some info....

I'm not suggesting that you were wrong, merely expressing my surprise. Maybe that make have a much more 'positive' connection between the internal cotacts and the manual operating lever. Most I've seen are designed so that the internal trip can operate even if one 'forces' the external operating lever to remain in the 'on' position. That obviously requires something less than a solid connection between the two, and means that (depending on design), internal tripping may (as Spark123 said) well not result in the external lever being able to operate something else which requires appreciable force (like a second MCB).

Kind Regards, John

 

[Spark123]

Well, as far as breaking stuff goes in the name of science here is a double poled MCB by Square D - same as Merlin Gerin which are both part of the Schneider group.

Untitled

• Spark123
• 8 Nov 2012
• 1

Notice the metal bar, this connects the two MCB trip mechanisms, the black dowels are just for locating them together along with 3 clips:

Untitled

• Spark123
• 8 Nov 2012
• 1

Even with the plastic bar between the levers on the front removed (and both MCBs put back together with the metal bar in place) both levers have to be reset together in order for them to hold up, resetting one results in it tripping immediately again. If both are reset simultaneously then switching one off causes the other one to trip.
Just hope the OP isn't after a 6A type C
(It did go back together again OK - honest boss)

 

[ban-all-sheds]

But without the more direct link between the two mechanisms, would one of them tripping cause the other one to switch off via the levers?

I'm assuming that the OP's electrician just linked the levers...

 

[Lectrician]

A TP MCB can be broken down to a 2 DP, but you should not use two SP to create a DP as (as above), the internal pins linking the mechs will be missing.

Merlin, Square D, old MEM etc can all be dismantled in an emergency!

 

[JohnW2]

But without the more direct link between the two mechanisms, would one of them tripping cause the other one to switch off via the levers? I'm assuming that the OP's electrician just linked the levers...

Exactly. As Spark123 (and then myself) said, it seems pretty unlikely that, at least with any make I've experienced, linking only of the levers would result in the desired functionality (or, at least, reliably result in the desired functionality).

Kind Regards, John