My first EICR and I'm confused! I'm industrial this is domestic

[Inteificio]

Hi,

I'm an industrial spark and taking my 2391 next week (wish me luck, I need it).

I decided to get a bit of practice and started an inspection on my own house.

I got as far as, b32 socket (ring?!) breaker looks slightly browned, better check it.

3 cables in the breaker two identical and one that appears to be a spur to an adjacent socket.

Q1) Let's start with the adjacent socket as that one should be easy:

It's a double socket.

There is no way a load can be put on that will stress the 2.5mm without blowing the bs1361s...however, the 2.5 is undersized to the b32.

clearly dodgy, but just curious how some experienced guys would treat this.


Q2) Ok now for the confusing bit.

The 'ring' is only a ring on the N. Live and E are open.
All sockets pass inspection (well small arcing on one N, which is fixed)
IR passes (on this circuit).

The board is split RCD so a borrowed N would take out the RCD.

All sockets have an in/out feed apart from one.

r1/r2 unsurprisingly no reading.

r1/rn would have been a bit more interesting, but haven't done yet.


Does anyone have any advice, this seems a strange one?

Thanks

 

[timhoward]

@Inteificio you have actually pin pointed (or stumbled onto?!) one of the interesting areas where the regs alone don't completely answer the question.
It reminded me of an interesting thread on another forum from years ago which I've attempted to link to.

IET EngX® Wiring Regulations Discussions

Questions on electrical systems design, electrical installations and BS7671 Wiring Regulations.

tinyurl.com

Plugging 800A fault current into the adiabatic with 0.1s disconnection time (the lowest the regs give us) gives a concerning answer, as you identified I think. But you need to read the bit in the box on the graph that says for larger PFCs use manufacturers data.

As above, we need a bit more info from the manufacturer to be reassured that the device will actually operate quicker than 0.1 s.
Manufactures will list a "Rated short circuit breaking capacity", i.e. it will cope with the fault current. At least 6kA usually.
And the manufacturers graphs usually go a lot lower than 0.1s. e.g. this Hager B16:


Plugging 800A into the adiabatic with 0.01s gives you a much more reassuring answer.
Hope that helps.

 

[Julie.]

Hi,

I've never actually come across this energy limiting class being used before. I only knew it existed as I wondered what the 3 on the front of the mcb was =)

Google was not kind with this. Its been discussed quite rarely and most of the time it's been abusing the guy asking the question, not helping! We've all got to learn sometime!

So Schneider puts class 3, 6k b20 as 45kA

Is it is simple as 4500=1200²xs.

S being the disconnection time?

Or have I guessed that all wrong?

If I am wrong anyone know anywhere that explains?

Thanks

It is probably an area that most forget, but the attachment here should help.

Ignore the "special" side of things, that is marketing, as if the other manufacturers don't do the same - they do!!

If you obtain the characteristics from the mcm manufacturer, it should both show how quick it actually trips, and the i^2t let through, which you can use to check coordination.


EDIT:
the characteristic posted by timhoward is typically of manufacturers' data.

At 40x it is going to be current limiting i.e. operates before the half cycle of 50hz completes - so actually less than 10ms.

Even if it doesn't current limit, it would clear on the first zero crossing, which is 10ms at 50hz & 8.33ms at 60hz

 

[pc1966]

Just as @Julie. says, you really need to get the manufacturer's data for more precise let-through values.

There are generic limits for this energy-limiting class of MCB and you see resulting limit values in, for example, Table B7 of the On-Site Guide. However, they are a touch pessimistic and most folks would just use the "standard circuits" of Table 7.1(i) to see if the combination of fuse or MCB, cable, and length are acceptable.

Here is an example taken from the Hager commercial catalogue technical information section for the B-curve MCB. What you see the the I2t let-through drop slowly as you go to higher thermal overload, then suddenly drop when you hit the "instant" magnetic trip region. Beyond that it increases as the PFC increases since the breaker does not open significantly faster with more overload.

But it does open a little faster, as a true "constant time" opening would be increasing at two decades of I2t for one decade of PFC (due to the I-squared part, with t=constant).

Behold:

 

[pc1966]

You can compute the equivalent "virtual opening time" from the I2t and I values. For example, taking the 6A MCB curve as easier to follow:

• At 30A PFC (Icc) the I2t is about 0.009k hence t = I2t / I^2 = (0.009 * 1000) / (30 * 30) = 0.01 = 10ms
• At 700A the I2t is about 1k hence t = (1 * 1000) / (700 * 700) = 0.002 = 2ms

In reality the opening is not instant after physical movement as it arcs, and there are other aspects to the MCB that limit peak current, but it serves to illustrate that an energy-limiting breaker opens in well under a cycle of the AC supply.

RCD are not energy-limiting in any real sense as they typically take a cycle or two of the mains to open (specification is below 40ms I think) and that is not significantly faster once you get beyond 5*In test current. Great for shock or smouldering fire prevention, but not helping much for a high current fault. In fact many domestic RCD are only rated to open 1.5kA which is below the sort of PFC you can see close to the DB. In these cases the manufacturer usually stipulates it should have a MCB that opens below 1.5kA (which is faster) so the RCD is not the one breaking a high fault current.

Even a 50A D-curve MCB meets this at 20*50A = 1kA but you will hardly see that is use domestically as it is hard to meet 5s disconnection Zs for a sub-main (compared to switched-fuse which is often better at selectivity with downstream MCBs), and even harder to meet the 0.4s for a final circuit.