The Dreaded C50 MCBS

[pete82uk]

Good afternoon all,

I was after a bit of advice, i have just came across some Crabtree C50's for the first time, i believe them to be the old 3kA type (white writing and an old installation).

The MCB is a 50A 3 Phase, the problem is we are installing a new distribution board in another part of the plant and the only place to take the supply from is this 50A C50. due to it only being 3kA, is there a way i can get past this?

i cant just simply remove this as its in an enourmous Distribution board.

i have looked at 434.5.1 which states : -

"except where the following paragraph applies, the rated short circuit breaking capacity of each device shall be not less than the maximum prospective fault current at the point at which the device is installed.

A lower breaking capacity is permitted if another protective device or devices having the necessary rated short circuit breaking capacity is installed on the supply side. In this situation the characteristics of the devices shall be co-ordinated so that th energy let-through of these devices does not exceed that which can be withstood, without damage, by the device(s) on the load side."

The problem i am having is knowing if using an MG (Schneider) Distribution board filled with Multi9 MCB's will meet this requirement.

i have spoken to electrium and all they have advised is i see if 434.5.1 can help.

any advice would be gratefully appreciated. Thanks in advance

Peter

 

[Sidewinder]

You really need the I^2t energy let through of the Schneider device upstream.

This should be available from Schneider.

You can then co-ordinate to this.

 

[sparkytim]

Have you measured the PFC at the DB?

 

[pete82uk]

Nope, i am an electrical design engineer (still in training), and at the moment the contracts only at the design stage. we wont be on site to check PFC etc until a month or so, however i need to have all my cable calculations complete for initial issue.

i will have a look into the I^2t of the schneider device sidewinder, cheers

 

[Chr!s]

I was after a bit of advice, i have just came across some Crabtree C50's for the first time, i believe them to be the old 3kA type (white writing and an old installation).The MCB is a 50A 3 Phase, the problem is we are installing a new distribution board in another part of the plant and the only place to take the supply from is this 50A C50. due to it only being 3kA, is there a way i can get past this?

i cant just simply remove this as its in an enourmous Distribution board.

Whats the PFC at that point in the installation ?

i have looked at 434.5.1 which states : -"except where the following paragraph applies, the rated short circuit breaking capacity of each device shall be not less than the maximum prospective fault current at the point at which the device is installed.

A lower breaking capacity is permitted if another protective device or devices having the necessary rated short circuit breaking capacity is installed on the supply side. In this situation the characteristics of the devices shall be co-ordinated so that th energy let-through of these devices does not exceed that which can be withstood, without damage, by the device(s) on the load side."

It wont tell you much.

Whats the expected PFC at the point of the new DB?

Id be careful using the let through energy values, where protective devices are in series, manufacturers info will be required to see what levels of current limiting are possible, your problem is you have two different manufactures.

---------- Post Auto-Merged at 14:53 ---------- Previous post was made at 14:27 ----------

What fuse/PD is upstream of the C50's, what standard of DB are they contained within?

 

[pete82uk]

The Distribution Compartment is protected by a switched isolator which house BS88 style fuses. its an large unit in the switchroom with approximately 40 compartments, all which house switched isolators for the compartment below. (usually 4 - 6 compartments per tier).

I have spoken to a few people now regarding this and been advised i may be better stripping out the compartment and installing a new DB inside, fed from the BS88 Fuses as this would give me what i require. the problems finding a compartment that is no longer in use as nothing can be turned off due to the type of works it is.

 

[platty668]

Do you have a reading for Zs at the distribution board Then simply calculate an approximate PFC as V/Zs if the value is significantly lower than the suspected 3ka breaking capacity then no problem. 434.1 PFC calculated measured or by enquiry.

What size are the BS88 Fuses on the supply side anything up to 200A should satisfy 434.5.1 as BS 88 200A has a prospective current 3000A at 0.1 secs Regs p.247

 

[pete82uk]

no reading yet, but i will try and get that sorted.

The BS88 Fuses are 125A which will satisfy the requirements. tried it in cable calcs and it sits happily.

Many thanks for everyones help.

 

[professional]

How on earth do you expect to be able to carry out ANY design calculations without any measured values to work with?

 

[Sidewinder]

Same way it has always been done by calculation!

What if the system does not yet exist, how do you design it then?

 

[professional]

Calculation based on what though?

If you have an existing installation you need measured values upon which to base your calculations.

 

[Sidewinder]

Well, that depends.

When I have discussed this with the engineers at my local DNO, they strongly recommend that I design to their default values rather than site values as they have an ongoing programme of network changes and upgrades, thus for example PFC may change following a network upgrade.

By the way I mean the network design and management engineers, not the jointers or linesmen in the field.

 

[SLIPSHOD & SLAPDASH]

Same way it has always been done by calculation!What if the system does not yet exist, how do you design it then?

Quite right, although when its an addition to an older designed installation like this then you have to know whats there & the values

 

[Manator]

Design is a wonderful thing, it allows you to break the mould, and stamp your mark. Everything a designer does has to be justified, and this is always done by calculation, which is why by design we are allowed diversity in design to items that are allowed no diversity in the OSG, for example.

Which is why, when you get drawings and specs for any installation, anything you are not sure of should be verified via the designer.

 

[Sidewinder]

Quite right, although when its an addition to an older designed installation like this then you have to know whats there & the values

Slips, I agree & disagree.

What is there now may not be there in a week.

You should design to DNO figures as changes are foreseeable, this must be allowed for.

You must contact the DNO or use the brb figures, there are other reasons that could take weeks to fo through!

 

[Chr!s]

Slips, I agree & disagree.What is there now may not be there in a week.

You should design to DNO figures as changes are foreseeable, this must be allowed for.

You must contact the DNO or use the brb figures, there are other reasons that could take weeks to fo through!

Sidewinder, i think what Slip Slap is saying will be the fact that if your connecting to equipment already installed, some knowledge will be required for design, how ever as you say if your connecting direct to the supply then its wise to work to the worst case scenario, this will cover any fluctuation in supply parameters.

 

[Sidewinder]

Plumber,

Yes information will be needed, not necessarily measurements though, even if not connecting direct to the mains.

You would need cable routes, sizes and protective devices then you would design accordingly based on DNO figures, as even though you are downstream, upstream changes will have downstream affects wil they not.

 

[Chr!s]

Plumber,Yes information will be needed, not necessarily measurements though, even if not connecting direct to the mains.

You would need cable routes, sizes and protective devices then you would design accordingly based on DNO figures, as even though you are downstream, upstream changes will have downstream affects wil they not.

Agreed Sidewinder