High psc

[Greyprof]

I have just been wiring a new conservatory and when I checked the Ze it came out as 0.02 ohms and the psc as 7.2kA. The existing MCB's are only rated as 6kA so I assume that they need changing to 10kA. A new MCB which feeds a small additional CU is already rated as 10kA so I assume that the MCB's in the additional CU will need to be rated at 10kA as well. What about the main switch in main CU and RCDs are they rated as well (nothing written on Eaton main switch, but 6000 written on RCD). hELP!!

 

[trix]

If you have protection from the rec, a 1361 type 2, and a consumer unit which incorparates a main switch to current B.S. you are allowed to rely on the breaking capacity of the 1361. Other factors are involved, but the general rule applies to domestic installations. This is in the reg's and the onsite guide.

 

[NoSparks]

If you have protection from the rec, a 1361 type 2, and a consumer unit which incorparates a main switch to current B.S. you are allowed to rely on the breaking capacity of the 1361. Other factors are involved, but the general rule applies to domestic installations. This is in the reg's and the onsite guide.

Can you point me to the right sections please since all I can find is the reference to backup cutouts which does not apply in this instance.

 

[ian.settle1]

If you have protection from the rec, a 1361 type 2, and a consumer unit which incorparates a main switch to current B.S. you are allowed to rely on the breaking capacity of the 1361. Other factors are involved, but the general rule applies to domestic installations. This is in the reg's and the onsite guide.

What a load of B*** S*** manufacturers of MCBs state that if the Max fault current rating of an MCB is exceeded then it should be replaced as irrepairable damage has been caused to the MCB and it cannot be guaranteed that it will operate in the future.

 

[iansparks]

Agree with ian.settle1! rely on RECs fuse

 

[ryanryan]

If you have protection from the rec, a 1361 type 2, and a consumer unit which incorparates a main switch to current B.S. you are allowed to rely on the breaking capacity of the 1361. Other factors are involved, but the general rule applies to domestic installations. This is in the reg's and the onsite guide.

Had this problem myself in a large industrial site with the pfc breaching 10kA, But was informed that the 30kA MCCB supplying the DB would provide the required level of breaking capacity..

 

[NoSparks]

Had this problem myself in a large industrial site with the pfc breaching 10kA, But was informed that the 30kA MCCB supplying the DB would provide the required level of breaking capacity

ryanryan, you are absolutly correct. MCCBs will disconnect the supply in 10ms (half a wave) and provided they are mached to the MCBs will afford adequate protection.

I have found NOTHING to support the concept that the energy let through of a BS1361 qualifies it as a suitable device to protect MCBs. I am still waiting for anybody to justify this claim.

 

[Des 56]

I trawled for this reply off the IET forum
This chap knows his onions

used in a domestic situation and connected to a single phase supply with a fault level of 16kA or less, and with a cut-out fuse no larger than 100A to BS 1361 type II - then

The UK has an A deviation to BS 60947 which effectively says that all Customer Distribution Boards used as described above have a conditional prospective fault current rating of 16kA - REGARDLESS of the prospective fault current rating of the individual mcbs or fuses.

This is how 1kA rewireable fuses have remained compliant with BS 7671. So in a house or similar - you do not need to check if the mcb prospective is greater than the actual prospective fault current - it always will be.

This does not apply in commercial or industrial installations or houses with supplies greater than 100A.


In industrial and commercial installations you do have check that the protective device has a high enough fault rating for the given fault level at the point in the system that it is to be installed.


I will tend to accept the above until better explanations are given

 

[trix]

What a load of B*** S*** manufacturers of MCBs state that if the Max fault current rating of an MCB is exceeded then it should be replaced as irreparable damage has been caused to the MCB and it cannot be guaranteed that it will operate in the future.

Not very helpful sunshine, I do respect your comments,,, yet not this one.


Des 56, cheers for the post;

As owners of good testers will realise through your excellent explanation, their anxiety levels have just plummeted.

 

[andyb]

Reg 434.5.1 covers this, and page 19 of the osg also.
So the consummer unit has to be to bs en 60439-3 or bs 5486, or protected by bs 88 or bs 1361 fuses then it is protected up to 16ka.

 

[NoSparks]

Thank you for your quotes.

434.5.1 States that

A lower breaking capacity can be provided ... In this situation, the characteristics of the devices shall be coordinated so that the energy let through of these devices does not exceed that which can be withstood, without damage, by the devices on the load side.

This applies to MCCBs coordinated in the laboritory with MCBs (see the ABB web site) but I am still searching for evidence that the energy let through of a BS1361 will not damage a standard 6000A MCB.

I have not yet been able to get hold of a copy of BSEN60439 or BS5486 which might shed some light on this. From 530.3.4 all consumer units must complying with BSEN60439 and specifically with the short circuit test defined in Annex ZA, in addition to 432.1 (which calls up 434.5.1)

In conclusion, until we find some evidence that the let through energy of a BS1361 will not damage a BS60898 rated at 6000A the BS1361 device cannot be assumed to give sufficient protection.

 

[andyb]

You have a good point NoSparks, but the fact is that I can't prove most things in the regs so I rely on the interpretation of others including the osg.
So if I have a high psc I use the osg, 3.3 on page 19.
If however my Zs for a particular circuit is over 6k, then I would ghange the mcb for a 10k one.

 

[NoSparks]

Well I had a Urika moment in the shower this morning.

The let through energy of a BS1361 will be given by the time it takes to blow (0.1 mS according to App 3 Fig 3.1 at currents in excess of 2000A) times the current passing (say 33kA) * voltage
volts x amps x Time = 230 x 33000 x 0.1 = 759,000 Joules. (0.75 Meg Joules)

The energy that a BS60898 MCB can stand is given by its own disconnection characteristics in Fig 3.4 which state that at currents in excess of 30 Amps (for a 6A device) the maximum disconnection time is 10.5 seconds. Also it is designed to withstand 6000 amps. So...

volts x amps x Time = 230 x 6000 X 10.5 = 14.49 Meg Joules (just to disconnect let alone damage)

Thus a BS60898 should be able to withstand the let through energy of a BS1361 .... as has been claimed!

The evidence was there all along - it just needed interpreting.

Now..... who is going to test the theroy?

 

[andyb]

Well I had a Urika moment in the shower this morning.

The let through energy of a BS1361 will be given by the time it takes to blow (0.1 mS according to App 3 Fig 3.1 at currents in excess of 2000A) times the current passing (say 33kA) * voltage
volts x amps x Time = 230 x 33000 x 0.1 = 759,000 Joules. (0.75 Meg Joules)

The energy that a BS60898 MCB can stand is given by its own disconnection characteristics in Fig 3.4 which state that at currents in excess of 30 Amps (for a 6A device) the maximum disconnection time is 10.5 seconds. Also it is designed to withstand 6000 amps. So...

volts x amps x Time = 230 x 6000 X 10.5 = 14.49 Meg Joules (just to disconnect let alone damage)

Thus a BS60898 should be able to withstand the let through energy of a BS1361 .... as has been claimed!

The evidence was there all along - it just needed interpreting.

Now..... who is going to test the theroy?

That's one hell of a shower you've got.