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MCB tripping characteristics, time-current curves #221170 02/11/21 12:20 PM
Joined: Feb 2021
Posts: 2
F
Foreigner Offline OP
New Member
Hi, all. I'm in the UK, so no familiarity with the NEC etc. But I do know that the laws of physics tend to be the same on both sides of the pond, and I thought that a breaker to any of the flavours of 60898-1 (IEC, EN, BS EN) would be the same the world over.

I came across this https://www.electricaltechnology.org/2020/02/calculate-circuit-breaker-size.html

and it says "a 30 amp circuit breaker will trip at 30 amp no matter if is it continuous or non continuous load".

Which from my POV is utter nonsense. Am I missing something? Is the site right? Over here a B32 will pass 36A for ever, and has to trip at 46.4A in less than 1 hour.

Or is what they say there utter nonsense?


Secondly, just below that they say "If we use a 100A circuit breaker for 30A circuit, it wont protect the circuit from fault currents".

Now, I accept that the NEC may not allow it, over here the Wiring Regulations do allow overload protection to be omitted in some circumstances, but fault protection has to be OK.

So even though it may not be compliant in the USA, are they right that a 100A breaker will not provide fault protection to a "30A" cable?

I don't know what fault current value and duration they are assuming.

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Re: MCB tripping characteristics, time-current curves [Re: Foreigner] #221174 02/11/21 06:15 PM
Joined: Nov 2005
Posts: 823
J
JoeTestingEngr Offline
Member
Welcome Foreigner!
It's as "Cold as Ice" here in Chicago.
You would have to look at the time-current curve for your series of breaker. They often don't show a trip time at the rating of the breaker. It might be never or hundreds of hours.
Overcurrent protection is what you're not getting with a 100A breaker and what would probably be a #10 wire in the USA. That would typically use a 30 Amp breaker. The fault current interrupting capability of a breaker is a different value, usually 1000s of Amps, describing the current that the breaker can safely interrupt. There are source impedance and motor load calculations used to determine the required values. Sometimes, current limiting devices are used to reduce available fault currents, and allow lower rated breakers.
Joe

Re: MCB tripping characteristics, time-current curves [Re: JoeTestingEngr] #221175 02/12/21 08:56 PM
Joined: Feb 2021
Posts: 2
F
Foreigner Offline OP
New Member
Originally Posted by JoeTestingEngr
Welcome Foreigner!
It's as "Cold as Ice" here in Chicago.

I can well believe that. Pick up your snow shovel and start walking south. When you get somewhere where you're asked "What's that?", stop walking yay


Quote
You would have to look at the time-current curve for your series of breaker. They often don't show a trip time at the rating of the breaker. It might be never or hundreds of hours.

I know.

An MCB to EN or BS EN 60898-1 with a rated current of In is required to not trip at up to 1.13In and must trip within 1 hour at 1.45In.

Which is why I was questioning what that website says.

"a 30 amp circuit breaker will trip at 30 amp no matter if is it continuous or non continuous load"

Because an EN 60891-1 breaker rated at 30A will not trip at 30A. Ever.

It's not a case of different makes maybe performing better - the standard requires a 30A breaker to pass 30A for ever.

It requires it to not trip at less than 39A. Ever.

Now - I thought the following:

1) The IEC 60898-1 standard was the same
2) The IEC 60898-1 standard was the one which applied to breakers used in the US.

In which case, "a 30 amp circuit breaker will trip at 30 amp no matter if is it continuous or non continuous load" is simply wrong. It would be here - I was trying to find out if it is there.


Quote
Overcurrent protection is what you're not getting with a 100A breaker and what would probably be a #10 wire in the USA. That would typically use a 30 Amp breaker.

That cable size makes sense - it's equivalent to 6mm², and our regs give that a current carrying capacity (for the type very similar to your Romex) of 23.5A - 47A, depending on how it is installed (i.e. factors such as is it in conduit, is it running through thermal insulation...)


Quote
The fault current interrupting capability of a breaker is a different value, usually 1000s of Amps, describing the current that the breaker can safely interrupt. There are source impedance and motor load calculations used to determine the required values. Sometimes, current limiting devices are used to reduce available fault currents, and allow lower rated breakers.
Joe

As I said, here, we are allowed to omit overload protection in some circumstances, and only provide fault protection.

Which isn't really anything to do with the breaking capacity of the protective device. Clearly that's a characteristic that can't be ignored when choosing a breaker, because you do want it to be able to break - many bad things will happen very quickly if the contacts weld together instead. And yes, upstream devices may limit the current the breaker has to interrupt - over here we're allowed to use 6kA rated breakers in domestic houses because there will always be a 60/80/100A fuse on the incoming supply. (Although I've heard that some electric companies are moving to 125A supplies for new houses to make them "EV ready" so it will be interesting to see if that 6kA type approval changes).

But that's not what I was asking about. If we take it as read that the breaker does clear the fault, the question then becomes does it clear it quickly enough to avoid the cable it is protecting from getting damagingly hot.

It isn't difficult to actually do the adiabatic calculation, the hardest part is gathering the data you need - what is the prospective short-circuit current? Are you going to ask the electricity company or measure it? What would the duration of that current be for your breaker? Do you get the maximum let-through energy data from the breaker manufacturer? Is the energy limited by an upstream device?

Returning to the example on that website "If we use a 100A circuit breaker for 30A circuit, it wont protect the circuit from fault currents", if we assume it is a #10 cable then assuming it's thermoplastic insulated/sheathed copper,according to my sums it can withstand a 476,100 A²s fault. As I said, I don't know what fault current and duration they were assuming when they made that pronouncement, but if we're looking at a breaker clearing a fault in 10s of ms, we're looking at fault currents of several '000A.

My perspective over here is that you don't have to be very far from your MV/LV distribution transformer for prospective fault currents to fall to levels where a 100A breaker will indeed provide fault protection for a 6mm² cable.

Do you guys measure/calculate prospective fault currents? Do you have a standard value you have to use?

I'm just not entirely comfortable with the accuracy of a blanket "If we use a 100A circuit breaker for 30A circuit, it wont protect the circuit from fault currents".

Overload currents? Of course it won't.

Fault currents? Hmmm.......


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