0.75mm Flex into 6A MCB

[uksparks]

Hi,

As per title, how would you feel about connecting a few garage lights onto some 0.75mm flex straight into the 6A MCB, ignore switching, just that its 0.75mm.

I have to say I normally go via a FCU but in this occasion I don't feel I need to as its in the garage with a 6A MCB I can come off.

referring to table 4F3A on page 378 it states 0.75 can take 6A.

id say I'm good to go, I k ow I'm probably stating the obvious but it's not something I usually do, hooking such a small co doctor up to an MCB.

reason for it being small is due to the fact I'm using 3 core and earth flex so I can use the switched output on the lights so they Interconnect on PIR.

 

[davesparks]

Slightly off topic, sorry UK! I never realised there are altitudinal limits on breakers!

View attachment 29429

Obvious when you think about it!

 

[NDG Elecs]

Obvious when you think about it!

Nope, not to me! All I can recall about the internals of mcbs is a bi-metal strip and possibly a solenoid.. Not from an engineering background so please enlighten me because it's bugging me now!!

 

[Andy78]

They should be available from most DB manufacturers, they won't be available for ranges exclusively intended for CUs though.

Can you use them for general/standard circuits? I see no reason why not, but undoubtedly someone will be able to come up with a reason why not.

7.5A was a reasonably common size once upon a time. I've seen a lot of 3871 MCBs in this size.

I love a 25A MCB myself. Not very common but so useful.

 

[davesparks]

Nope, not to me! All I can recall about the internals of mcbs is a bi-metal strip and possibly a solenoid.. Not from an engineering background so please enlighten me because it's bugging me now!!

You'll be surprised how many things are rated to work within specified altitudes. I've drink too much to discuss it at the moment
But basically it is due to the change in atmospheric pressure causing it to operate outside of limits

 

[NDG Elecs]

You'll be surprised how many things are rated to work within specified altitudes. I've drink too much to discuss it at the moment
But basically it is due to the change in atmospheric pressure causing it to operate outside of limits

Cheers Dave, I realise many things do have various limitations. Just never read that particular one before! Just been googling it and found nothing specific, but my money is on the coil/solenoid/hammer trip part being the reason. I would imagine a bimetallic strip would perform above 2000M.

 

[Marvo]

The internal heater that operates the bi-metalic strip produces a certain amount of heat at a certain current. The bi-metalic strip is designed to distort according to the heat being absorbed from the internal heater.

At sea level the air is quite dense and it has a certain cooling effect the removes a certain quantity of heat from this internal mechanism but at altitude the thinner air doesn't cool the mechanism as efficiently so more of the heat being produced is retained internally causing inaccuracies to occur.

The higher altitude causes the tripping curve of the mechanism to drift so your C curve MCB starts behaving more like a B curve. Many MCB's would have an altitude derating factor available from the manufacturers on request.

 

[Andy78]

The internal heater that operates the bi-metalic strip produces a certain amount of heat at a certain current. The bi-metalic strip is designed to distort according to the heat being absorbed from the internal heater.

At sea level the air is quite dense and it has a certain cooling effect the removes a certain quantity of heat from this internal mechanism but at altitude the thinner air doesn't cool the mechanism as efficiently so more of the heat being produced is retained internally causing inaccuracies to occur.

The higher altitude causes the tripping curve of the mechanism to drift so your C curve MCB starts behaving more like a B curve. Many MCB's would have an altitude derating factor available from the manufacturers on request.

The same effect that causes water to reach boiling point at lower temperatures at higher altitudes ?

 

[NDG Elecs]

Cheers Marv, good job I aint a betting man eh!!

I always thought that overcurrent flowing through the bimetallic strip caused the bend and break. Did not realise there was another mechanism at play ie the heater. Is that the arc chutes?

Am I correct in thinking the bimetallic protects better for gradually overloaded ccts and the solenoid for more instantaneous overcurrent.

 

[Marvo]

The water boiling point thing is more about the reduced atmospheric pressure than the actual cooling effect reduction by the air being thinner but it's all kinda linked in a roundabout sort of way.

 

[Andy78]

The water boiling point thing is more about the reduced atmospheric pressure than the actual cooling effect reduction by the air being thinner but it's all kinda linked in a roundabout sort of way.

Well my physics thinking may be dulled by a bottle or two of shiraz. I was just hoping it would be that simple. Just say yes

 

[NDG Elecs]

Part of my degree course covered atmospheric science...but I was mainly in the pub all the time so I remember diddly squat of it!!

 

[NDG Elecs]

Well my physics thinking may be dulled by a bottle or two of shiraz. I was just hoping it would be that simple. Just say yes

In a nutshell bubbles have less opposition!!!

pressure - Explanation for different boiling points of water on different altitudes - Physics Stack Exchange

 

[Marvo]

Cheers Marv, good job I aint a betting man eh!!

I always thought that overcurrent flowing through the bimetallic strip caused the bend and break. Did not realise there was another mechanism at play ie the heater. Is that the arc chutes?

Am I correct in thinking the bimetallic protects better for gradually overloaded ccts and the solenoid for more instantaneous overcurrent.

Yep, there's two separate tripping mechanisms in an MCB, I was discussing the thermal mechanism which caters for partial overload scenarios, the magnetic mechanism caters for overloads of 5 times the rated current or more and this operates in a completely different way.

The weakness in the thermal system is because the mechanism isn't operated directly by current, there's a couple of steps in between where the discrepancies creep in. The load current firstly causes a small heater to produce heat, this heat is then transferred to a bi-metalic strip, the strip deflects and this deflection is what causes the mechanism to trip. Each of these steps in the chain provides scope for inaccuracies to creep in and altitude is an external factor that takes the inaccuracies beyond the point where they can be ignored. Ambient temperature would be another external factor that could require compensation being applied by using a derating factor as well, in extreme cold conditions a C curve MCB starts behaving more like a D curve. This isn't just an MCB issue, same applies for thermal overloads in DOL starters for example.

Arc shutes just contain the flash that occurs when contacts are separated whilst they're under load.

 

[Andy78]

In a nutshell bubbles have less opposition!!!

pressure - Explanation for different boiling points of water on different altitudes - Physics Stack Exchange

Ah now that makes more sense. I was thinking there was a general relationship between temperature within matter and atmospheric pressure.
But then, I haven't ever studied physics past school level and I have had much wines

 

[NDG Elecs]

Yep, there's two separate tripping mechanisms in an MCB, I was discussing the thermal mechanism which caters for partial overload scenarios, the magnetic mechanism caters for overloads of 5 times the rated current or more and this operates in a completely different way.

The weakness in the thermal system is because the mechanism isn't operated directly by current, there's a couple of steps in between where the discrepancies creep in. The load current firstly causes a small heater to produce heat, this heat is then transferred to a bi-metalic strip, the strip deflects and this deflection is what causes the mechanism to trip. Each of these steps in the chain provides scope for inaccuracies to creep in and altitude is an external factor that takes the inaccuracies beyond the point where they can be ignored. Ambient temperature would be another external factor that could require compensation being applied by using a derating factor as well, in extreme cold conditions a C curve MCB starts behaving more like a D curve. This isn't just an MCB issue, same applies for thermal overloads in DOL starters for example.

Arc shutes just contain the flash that occurs when contacts are separated whilst they're under load.

Now if there is no plagiarism in the above I applaud you!! When I looked at an mcbs internals I saw the arc chutes, and like the name implies I thought they were for what you said. Then you threw a heater into the equation and I went off at a tangent!

Where is the heater, presumably adjacent to the bimetal strip... Cheers fella.