I need something explaining!

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sorry peeps, 'Pache can explan this bit if he is around I'm confidentEnergy = Energy, POWER = POWER, so no matter what!
My simplified understanding is energy [in joules] is the capacity to do work. Useful to remember it can neither be created or destroyed, just change state [electrical energy eg in a motor to kinetic energy AND heat [losses].

Power [in watts] is the rate of use of energy [or doing work]. So to use 1 joule of energy for 1 second uses 1 watt. Or if you like 1W = 1J/s.

How thorough do you want me to be with my answer sir? I can try and throw in power factors buy my trigonometry is rusty so I may have to google the formulae!

 
BHP = PFC

torque = how fast it gets there

any easier for you,

if you have a low bhp but a high torque you will hit the limit quicker,

the torque overcomes the resistance, and if you also have a very low restistance then you get there even faster, this is why racers change the gearing so much, to get the torque&bhp to match the achievable speeds they need,

and we use the same principle to select protective devices to allow necessary current through at acceptable speeds and still achieve protection, rating, type and pfc of MCBs.

more torque=less resistance

more bhp= higher PFC

 
OK we now know from previous posts

That slow releases of energy are safe

The relationship between power & energy

That if an mcb is subjected to a fault current in excess of its rated breaking capacity its then goosed

That IF an mcb (or fuse) is subjected to a fault current well in excess of its rated breaking capacity then it may not break the fault current and may well ionise and cause the fault to continue destructively until a weak point upstream of this location fails, ignoring for now the DNO fuse, this may be the ACB in the local sub unless there is a weak cable joint between this and the fault

That the a.c. mains in the UK has zero current flow several times per second

That mcb's have 2 modus operandi, thermal & magnetic, hence the differing shapes of the curves in the brb and their other name TM breakers

When a current is interrupted the voltage will ionise the air but the current is required to sustain the arc, all being equal this arc will extinguish normally within fractions of a second

The addendum to the CU standard simply relies on the design and construction of the unit and the upstream backup device being a 1361/88 or larger CB capable of interrupting the full fault current

I have not read the standard as far as I can remember, and tonight will not be the night

I have today

 
So to put it simply a 1361 cuts the supply quickly and prevents damage to downstream MCBs

 
Thanks and I do enjoy your detailed input.

The "Meat" is great, but I find sometimes its good to see the bones clearly to understand things.

;)

 
so in the case of the 6ka mcbs, they would be rendered useless at 9ka but would they have caused danger by setting on fire or something. If not then you only risk having to replace a single mcb if this very rare occurrence happens. Also what happens to a semi-enclosed fuse if this has to take 9ka? can they be scraped out and re-used?

 
With a fuse the fauly element becomes a plasma ball, the risk of this is that it can ignite items in the vicinity, this is limited often by asbestos lining, remember that magic material that would not catch fire and was an absolutely fabulous thermal insulator!

An mcb can "weld" closed as it is a mechanical device with moving parts, then it relies upon pstream protection to limit the fault current and the energy release / let through.

The welded mcb can allow huge currents to flow causing a lot of collateral damage.

 
I have looked at some manufacturers data including wylex that shows on a graph that if say there is a PFC of 6Ka, the current will get no-where near that, before switching off. The way i see it, if the MCB is rated at say 6Ka and there is a PFC of say 9Ka, there should be no problem still as the mcb would not let through anywhere near 9Ka to damage the mcb if it is working properly. The back up fuse seems to me just that, a back up if the mcb turns out to be dodgy, but if working correctly, this back up would not be used as the mcb would not let enough energy through anyway.

---------- Post Auto-Merged at 21:40 ---------- Previous post was made at 21:22 ----------

Eaton Electrical United Kingdom | Eaton branded - Miniature Circuit Breakers

see page 8, It shows a graph where the connection is interupted well before the PFC, but shows that an arc still carries electric on after it switched off.

 
I guess from your response, that you didnot bother to look at the graphWet Fish

---------- Post Auto-Merged at 18:26 ---------- Previous post was made at 18:24 ----------

Its on the mcb pdf option to the right

 
ok so it must be rubbish then cos you dont like Eaton?????

(on the mcb protective device product guide pdf to the right)Wet Fish

 
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