Socket Breaker

[spark 68]

Don't really think that any extra information is required.
The OP is really just asking what type of MCB should be used for general circuits.
Unfortunately, they've gone about asking the question in a rather round about fashion.
The answer of course being Type B.

I said that last night lol

 

[GEOFF S]

Under BS7671 wiring regs.

Wht are you not working to the NEC requirements, overcurrent is dealt with a little differently, maybe explain to us how you provide protection through NEC requirements, im sure many would be interested.

Cheers

 

[Gardner]

Don't really think that any extra information is required.
The OP is really just asking what type of MCB should be used for general circuits.
Unfortunately, they've gone about asking the question in a rather round about fashion.
The answer of course being Type B.

I see it in a similar light. My concern is that while a B type will work, certain circuit factors like long runs can increase trip time which would violate code. If I know where to look, or the equations I can compute the values based on known factors and use an EFLI tester to confirm the values.

Wht are you not working to the NEC requirements, overcurrent is dealt with a little differently, maybe explain to us how you provide protection through NEC requirements, im sure many would be interested.

Cheers

Glad you asked. Basically the NEC is more like a shopping list or design guide. If we have X, NEC says reach for Y. If we have A then we pick B, at least the overall structure of the code. Earth fault loop impedance does not exist. For most circuits we just pick a breaker based on the current needed and make sure the wire is sized correctly. Type A, B, C ect breakers do not exist. Most breakers are just 10X to 15X (a few like 15 and 20amp single pole square D breakers are 5 to 6x) the handle rating for instantaneous trip and that works in most cases (basically the equivalent of your highest magnetic trip MCBs but just used everywhere) Larger breakers over 150 or 200 amps generally come with adjustable trip curves for coordination purposes, but few are willing to spend the money for a coordination study so the dials are either left turned down (fault on branch circuit pops the main as well) or are turned all the way up (a fault on a wire means major arc flash/fire cracker time) .

Here is an interesting link on that actually you guys will love LOL: http://paceforensic.com/pdfs/Circuit_Breakers_The_Myth_of_Safety.pdf

For general use circuits in the USA we use 2.08mm2 wire (14 guage) and 3.31mm2 (12 gauge) protected at 15 and 20amps respectively. No equations exist or higher current ratings like if the wire is clipped directly to a wall.

For the earth wire we just select what article 250 tells us to based on breaker and wire size.

14 gauge, means 14 gauge earth wire

12 gauge, earth wire means 12 gauge earth wire

10 gauge, equals 10 gauge earth wire

8 gauge and 6 gauge, means 10 gauge earth wire

4 gauge means 8 gauge earth wire ect.

Not very technical, but its seen to suffice.

However, in BS7671 my understanding is that earth wires can be smaller than L or N, (such as 2.5mm2 twin with 1.5mm2 earth) but must pass enough current to trip the breaker in 0.5/50 of a cycle. In this regard a difference exists between the NEC and BS7671.

 

[davesparks]

You need to select the type and rating of breaker which is correct for the load being supplied, then you select a conductor size from that. If the earth fault loop impedance is too high then you select a larger conductor size to reduce it.

Earth wires can be smaller yes, the actual size of an earth wire required is calculated using the adiabatic equation.
Selection of earth wire size can alternatively be done in accordance with a table which basically says:
for a line conductor of 16mm csa or less the earth must be the same size
for a line conductor of either 25 or 35mm the earth must be 16mm
for a line conductor of 50mm or greater the earth must be at least half the csa of the line

The calculation method will invariably lead to a smaller earth being acceptable (subject to EFLI considerations)

 

[Engineer54]

Wht are you not working to the NEC requirements, overcurrent is dealt with a little differently, maybe explain to us how you provide protection through NEC requirements, im sure many would be interested.

Cheers

As i remember, the OP's company are manufacturing Mobile/Portable cabin like structures that are being exported to the UK, and therefore needs to be wired to in accordance to BS 7671 for compliance purposes...

 

[spark 68]

As i remember, the OP's company are manufacturing Mobile/Portable cabin like structures that are being exported to the UK, and therefore needs to be wired to in accordance to BS 7671 for compliance purposes...

Which was precisely why I asked for more information ie. circuit length and installation method etc., if this is to be *fully* certified for UK use then by rights it will need to take voltage drop into account too.

Sure we can give him standard circuit data/layout for basic radial S/Os MCB and cable sizes, but how are we to know if it is correct without much more information ?, then there is the unknown supply arrangement to consider, also other rules may be required to supplement this, example PME and metal clad buildings, or mobile structures, flexible cables only etc...etc. and on it goes.

 

[Engineer54]

Which was precisely why I asked for more information ie. circuit length and installation method etc., if this is to be *fully* certified for UK use then by rights it will need to take voltage drop into account too.

Sure we can give him standard circuit data/layout for basic radial S/Os MCB and cable sizes, but how are we to know if it is correct without much more information ?, then there is the unknown supply arrangement to consider, also other rules may be required to supplement this, example PME and metal clad buildings, or mobile structures, flexible cables only etc...etc. and on it goes.

Supply wise there isn't much he or his company can do about that, ...that would also go for a UK manufacturer, therefore they can only conduct meaningful dead tests at the factory. It would then be down to the electrician that is providing the supply to a portable type building, to conduct live tests etc. These porta type cabins may only be in place 6 weeks or 6 months before being moved to another location...

 

[spark 68]

Supply wise there isn't much he or his company can do about that, ...that would also go for a UK manufacturer, therefore they can only conduct meaningful dead tests at the factory. It would then be down to the electrician that is providing the supply to a portable type building, to conduct live tests etc. These porta type cabins may only be in place 6 weeks or 6 months before being moved to another location...

That was partly my point, I can give him max Zs values, but without a known Ze it is a bit pointless unless they are maybe using an upfront RCD as per caravans which would give a lot more leeway, I could also give him max circuit lengths for a given csa/MCB combination, but without knowing the supply parameters this would also be a bit pointless at the moment.

There are no doubt rules of thumb for designing such layouts, that would go for a manufacturer here too, but without detailed knowledge of how this "thing" is constructed, dimensions and so forth and also intended use etc. none of us can really design this for him from an internet forum.

If the OP gives us more details between us we could probably suggest some solutions for him to consider, that is if sparks can ever agree on anything lol

 

[Gardner]

You need to select the type and rating of breaker which is correct for the load being supplied, then you select a conductor size from that. If the earth fault loop impedance is too high then you select a larger conductor size to reduce it.

Earth wires can be smaller yes, the actual size of an earth wire required is calculated using the adiabatic equation.
Selection of earth wire size can alternatively be done in accordance with a table which basically says:
for a line conductor of 16mm csa or less the earth must be the same size
for a line conductor of either 25 or 35mm the earth must be 16mm
for a line conductor of 50mm or greater the earth must be at least half the csa of the line

The calculation method will invariably lead to a smaller earth being acceptable (subject to EFLI considerations)

My thoughts exactly. An equation would factor in lenghth and the normal resistive ohms (Inductive {reactive} impedance can usually be ignored for short runs like this) to come up with maximum current anticipated for a fault, and should that current on the furthest end of the spur exceed the instantaneous trip the earth wire size is increased.

I read somewhere in BS7671 that an option to increasing the earth wire size in increased disconnect times, is equal potential bonding around the area of the device to reduce the touch potential between it and other conductive objects, however, in this case its best that not be considered.

I will be avoiding any ring circuits FWIW, as I do not feel confident in there use since a break in the ring will not be caught by any MCB or RCD.

 

[Engineer54]

That was partly my point, I can give him max Zs values, but without a known Ze it is a bit pointless unless they are maybe using an upfront RCD as per caravans which would give a lot more leeway, I could also give him max circuit lengths for a given csa/MCB combination, but without knowing the supply parameters this would also be a bit pointless at the moment.

There are no doubt rules of thumb for designing such layouts, that would go for a manufacturer here too, but without detailed knowledge of how this "thing" is constructed, dimensions and so forth and also intended use etc. none of us can really design this for him from an internet forum.

If the OP gives us more details between us we could probably suggest some solutions for him to consider, that is if sparks can ever agree on anything lol

Seeing as they don't possess a loop testers in the States, it would all be down to calculation anyway..lol!!

 

[Engineer54]

http://paceforensic.com/pdfs/Circuit..._of_Safety.pdf

That pdf link is circa 1984, so has anything changed regarding the contents of this article, eg design of your MCB's magnetic trip mechanism?? Or is the introduction of the so-called AFCI's been the answer to the arcing problem??

 

[Gardner]

That pdf link is circa 1984, so has anything changed regarding the contents of this article, eg design of your MCB's magnetic trip mechanism?? Or is the introduction of the so-called AFCI's been the answer to the arcing problem??

Other than manufacturers opting to set the magnetic trip at 10x the handle rating (6x for Square D), rather than well over that like in the past, not much has changed. AFCIs are supposed to be the "solution" to a lot of problems, but in truth just a gimmick. An RCD breaker (RCBO) with a low magnetic trip does just about everything an AFCI does. In fact the only thing an AFCI can do over that (at least claimed) is a series arc fault. But at 120 volts unless vibration or tugging exists on the loose connection sustaining an arc at 120 volts is nearly impossible.

To this day, the biggest contributor to electrical fires in the US and around the world are glowing connections. If a device could be put in place to stop them then that would be a real product investment unlike AFCIs.

The good new is that about 80% of AFCIs have 30 to 50ma RCD built in, and that has caught a lot of wiring errors that insulation testing would have in the first place.