High PFC. Cascading

[Sid]

Hi fellas,

Doing a new build at moment, its had a new supply put in - Ze is 0.02 ohms, Pfc is 12kA. Besides changing all Mcbs for higher rated breaking capacity are there any other methods that could be used to make this install comply with regs? I asked a colleague who suggested applying a calculation for cascading to the additional notes section of EIC. Is anyone familiar with this method or calculation - had a look on line but finding it a bit confusing tbh?

Any constructive advice would be appreciated. Thank you

 

[ImpededLoop]

If the DNO fuse has a high enough kA rating, you don't need to worry about the downstream devices.

I'm sure there will be conflicting opinions on this though!

The property must be right next to the transformer to get a Ze that low.

 

[Marvo]

Doesn't the CU as a pre-assembled unit come with some kinda fairytail PFC rating above the individual kA rating of the MCB's?

 

[spark 68]

A consumer unit to BSEN604393-3 with overcurrent protective devices up to 45A on a SP supply protected by a 100A fuse or less is conditionally rated as 16kA, this means the whole assembly including the breakers/fuses contained within may taken as 16kA.

There is an annexe ZA that explains this in more detail, someone may have already posted this annexe on here.

 

[Silly Sausage]

Could someone clarify this question once and for all, in nice simple terms!
I'm thinking that the lower rating device can blow itself to smithereens, but not necessarily disconnect the power; however, as long as an upstream device is capable of handling the fault current and is guaranteed to disconnect the power, all is good in the world?

 

[Marvo]

So they're basically declaring the inside of the CU a fault free zone to all intents and purposes.

 

[Marvo]

Could someone clarify this question once and for all, in nice simple terms!
I'm thinking that the lower rating device can blow itself to smithereens, but not necessarily disconnect the power; however, as long as an upstream device is capable of handling the fault current and is guaranteed to disconnect the power, all is good in the world?

You're looking at the difference between Icn and Ics ratings of disconnection devices.

 

[spark 68]

I think it is because the further you get from the source the PFC decreases very rapidly Archy, the protective devices themselves also have fairly high impedances at the kA level, and also the outgoing cables to these protective devices are relatively small compared to the ones at the origin, then you have the speed of operation of these devices.

Also another factor is normal MFTs cannot accurately measure this low impedance, recall that in the PFC calculation a small change in Zdb or whatever has a much larger effect on the final PFC figure.

 

[Silly Sausage]

68, Aye, I can see what you're saying there.
Have to do some web digging and reading!

 

[Leesparkykent]

A consumer unit to BSEN604393-3 with overcurrent protective devices up to 45A on a SP supply protected by a 100A fuse or less is conditionally rated as 16kA, this means the whole assembly including the breakers/fuses contained within may taken as 16kA.

There is an annexe ZA that explains this in more detail, someone may have already posted this annexe on here.

I think This only applies if protected by a BS1361 fuse up to 100amp though?

 

[GEOFF S]

You can't assume back up protecion is afforded just because the upstream device has a suitable breaking capacity, it doesnt work like that.

To Cascade you need to know all the fault levels through out the system. You will also need to use the manufactures charts to be successful.

Basically if the uptream device limits the current to a value which the downstream device can handle then back up protection is afforded.

Uk Annex ZA basically utilises this, the current is limited by the BS88, the Cu will never see 16kA.

Cheers

 

[Engineer54]

You can't assume back up protecion is afforded just because the upstream device has a suitable breaking capacity, it doesnt work like that.

To Cascade you need to know all the fault levels through out the system. You will also need to use the manufactures charts to be successful.

Basically if the uptream device limits the current to a value which the downstream device can handle then back up protection is afforded.

Uk Annex ZA basically utilises this, the current is limited by the BS88, the Cu will never see 16kA.

Cheers

Careful there, it very much depends on how close the DB/CU is to the source of the supply. For arguments sake if a final circuit DB/CU is located within a main switchboard room next to the source TX, to say supply the lighting and small power circuits for the area, everything could change... We always install 22/25KA MCB's in such circumstances.

What you are describing in your 2nd and 3rd paragraphs above, is called a ''Protection Coordination Study'' and will cover both MV and LV protection devices from the TX/ Main switchboard all the way through to a sample final circuit on the furthest final circuit DB/CU. This is probably far beyond the scope of most electricians, but is common practice if not a requirement on the larger projects with extensive electrical distribution systems.

 

[Knobhead]

Which is why we never fitted “small power” feeds off a main board unless a riser fuse was fitted.

With 50+KA as the prospective fault level and a 16KA MCB as first line protection……...?

It isn’t going to end well.

 

[Engineer54]

Which is why we never fitted “small power” feeds off a main board unless a riser fuse was fitted.

With 50+KA as the prospective fault level and a 16KA MCB as first line protection……...?

It isn’t going to end well.

Worse, most MCB's will usually be 6KA or 10KA, most manufacturers don't produce the higher KA ratings.

Initial fault levels will be generally be taken care of by the supplying MCCB, but there is always the let-through to be considered when high source fault levels are a factor.

 

[ian.settle1]

Worse, most MCB's will usually be 6KA or 10KA, most manufacturers don't produce the higher KA ratings.

Initial fault levels will be generally be taken care of by the supplying MCCB, but there is always the let-through to be considered when high source fault levels are a factor.

MEM are making a 15KA rating in their MEMSheild 3 series now