I need something explaining!

[Chr!s]

Could someone please explain to me, in really plain English, how the theory regarding 'let through' energy ratings of BS1361 main fuses, upstream of MCBs, affects the max PFC ratings of these MCBs.

Thats what ive explained for you, in simple english.

Hi Plumber,Good explanation - but is this 'back-up' in case of failure of the MCB to open under fault conditions?

Because surely it will open well before the max PFC is reached, at which point that will be the end of the fault current anyway.

I am, of course, assuming that a 16 kA fault current doesn't just appear and actually builds up to the max - but either way, surely the MCB will clear the fault well before the maximun, without any need for the 1361 to blow?

Its got nothing to do with mcb failure?

Your question was realting to back up protection, now the question in bold is relating to discrimination, which is a little different.

 

[Chr!s]

Is that right, surely backup protection is afforded in both scenarios? Or maybe now we've gone all schoolesque some of the terminology has become a bit wacky!

Its right, i can assure you.

In effect the pfc at the 6ka MCB is 9ka which is more than it can break safely

 

[Lurch]

Its right, i can assure you.In effect the pfc at the 6ka MCB is 9ka which is more than it can break safely

That's what I mean, maybe I'm reading it wrong, but to me afforded means that backup protection is available, so it is true in both cases, regardless of the actual PFC.

 

[Chr!s]

That's what I mean, maybe I'm reading it wrong, but to me afforded means that backup protection is available, so it is true in both cases, regardless of the actual PFC.

When i say afforded i mean in that instance the mcb rating is adequate due to the current limiting of the upstream fuse

 

[sparkytim]

Ok another quote for the topic;

ESC Essential Guide Topic F13-25 7.1

"For overcurrent protective devices incorparated in a Consumer Unit complying with BSEN 64439-3 1991 current not exceeding 50 A the rated short-circuit capacity may be taken as 16 KA - conditional on the Consumer Unit being protected by a BS 1361 Type 2 Fuse not more then 100 A "

 

[ADS]

Thats what ive explained for you, in simple english.

No, you see. You haven't.

You have given us scenarios that work and scenarios that don't - that doesn't explain HOW they work and HOW they don't.

How does the 1361 stop a fault current reaching the maximum?

How does it work??

Lets say we have a short circuit in scenario 1.

So walk us through it.

Fault current starts to build up - hits the amount required to trip MCB in 0.1 sec and MCB trips - doesn't it??

Or is the maximum fault current instant - in that case, surely the MCB will still trip - even if not 'safely'....regardless of what the 1361 does?

See, this is the part I'm not getting - and this is the part I wanted in 'plain English'.....a walk through of what happens.

Its got nothing to do with mcb failure?

Then the MCB will trip regardless, surely.

 

[Chr!s]

See, this is the part I'm not getting - and this is the part I wanted in 'plain English'.....a walk through of what happens

Although not difficult, its not easy in plain english lol

 

[ADS]

I know - it does my head in

Some MCB manufacturers have some good literature on it, but I'm still struggling to grasp it.

 

[Sidewinder]

ADS,

Have you got your head around the energy thing in my 1st post?

If so then we will move on.

 

[spinlondon]

AC current is produced in GB at a cyclic frequency of 50Hz.

This means that the current goes from zero to upper peak, back down through zero to lower peak and back to zero 50 times a second. At 50Hz, each cycle takes 0.02 seconds. Peak current is reached in 0.01 seconds.

HRC fuses will operate in less than half a cycle, before peak current is reached, thus limiting the current that reaches more sensitive equipment such as MCBs.

 

[steptoe]

and if it reaches above the maximum fault current of the protective device faster than the protective device can break it(Zs) then it will in effect Weld the MCB closed.

 

[ADS]

AC current is produced in GB at a cyclic frequency of 50Hz.This means that the current goes from zero to upper peak, back down through zero to lower peak and back to zero 50 times a second. At 50Hz, each cycle takes 0.02 seconds. Peak current is reached in 0.01 seconds.

HRC fuses will operate in less than half a cycle, before peak current is reached, thus limiting the current that reaches more sensitive equipment such as MCBs.

This kind of makes sense, Spin, but won't the MCB have interupted the fault current by this time?

Otherwise the 1361 will operate all day long instead of the MCB.

 

[ADS]

and if it reaches above the maximum fault current of the protective device faster than the protective device can break it(Zs) then it will in effect Weld the MCB closed.

Which also makes sense, which is why I mentioned MCB failure earlier.

 

[ADS]

ADS,Have you got your head around the energy thing in my 1st post?

If so then we will move on.

OK , sidewinder, just re-read it - continue, please

Sorry about the multiple posts:Blushing

 

[Sidewinder]

OK next we need to establsh the definition of & the difference between power & energy.

I am kinda hoping that Apache will do this one as I'm busy for an hour or 2, 'pach, could you?

 

[spinlondon]

This kind of makes sense, Spin, but won't the MCB have interupted the fault current by this time?Otherwise the 1361 will operate all day long instead of the MCB.

Unfortunatley no.

Most MCBs have 2 types of operation, a thermal overload and an electromagnetic one.

The thermal overload is a Bi-metalic strip, which will gradually bend as it overheats, untill it eventually trips.

The electromagnetic is generally by way of a solenoid, which is set to operate at a pre-determined load. This operates in a tenth of a second (0.1) which will allow 5 cycles of current to occur. Peak voltage will be reached 10 times.

If the peak current is above the breaking current of the MCB, the MCB will be destroyed, often quite spectacuarly.

The MCBs operate instead of the HRC fuse, because it is very rare that a zero impedance fault occurs. Any impedance along the fault path will limit the current to a degree. The greater the impedance, the greater the current limitation.

 

[kme]

Ok another quote for the topic;ESC Essential Guide Topic F13-25 7.1

"For overcurrent protective devices incorparated in a Consumer Unit complying with BSEN 64439-3 1991 current not exceeding 50 A the rated short-circuit capacity may be taken as 16 KA - conditional on the Consumer Unit being protected by a BS 1361 Type 2 Fuse not more then 100 A "

From January:

http://www.talk.electricianforum.co.uk/showpost.php?p=102115&postcount=13

Was in "connections" mag, last year......

:innocent

KME

C`mon snakehips - interest is piqued - need something to clear the boredom....

(p.s. FYI - it is NOT a "premier travel inn"; its a soddin "travel lodge".).

Carp food (services), no alcohol (luckily I brought me own), but its 10 mins from the training centre..................

 

[Sidewinder]

Mr. KME, SIR, that is what I was hoping to have explained to you.

Travel Lodge = CARP!

I'll have to carry this on tomorrow, still have an engineering report to finish, sorry peeps, 'Pache can explan this bit if he is around I'm confident

Energy = Energy, POWER = POWER, so no matter what!

 

[steptoe]

if anyone wants to make a comparison to an engine its similar to BHP and TORQUE

if you can understand how that works then you will have an easier time understanding this.

torque = energy

BHP = power

 

[spinlondon]

My motorcycle engine has high torque, but quite a low BHP.

What does that mean?