PFC reading

[zsoltimatrix]

Hi guys,

I just finished some work, 3 phase motors on a new distribution board. Measured the PFC which comes out as 40KA (two times 20KA measured since it is three phase).

Now which devices should clear this value? The isolator in the main distribution board which is supplying the new distribution board? Or the devices in the new distribution board for which I measure the PFC at the incoming side?

Thanks!

 

[dichroic]

BS 7671 requires the following prospective fault currents to be determined for each installation by calculation, measurement or enquiry:
• the prospective short-circuit current at the origin (Regulation 313.1 refers), and• the prospective short-circuit and the prospective earth fault current at every relevant point of the installation (Regulation 434.1 refers).

Regards

Dichroic

 

[zsoltimatrix]

Exactly.

I measured 20KA between L-N so for 3phase it is 40KA.

But, Ze is 0.02 ohm. So by calculation it should be around 20KA, isnt it?

My MCB`s are only 10KA reated, looks like i will have to change them.

 

[marcshaw]

I think, as long as you have a device in circuit, including the main cut out fuse that can break the KA reading you should be fine. Also your 40 KA is that at origin? If you test at the MCB output terminal what is the KA there?

 

[zsoltimatrix]

I think, as long as you have a device in circuit, including the main cut out fuse that can break the KA reading you should be fine. Also your 40 KA is that at origin? If you test at the MCB output terminal what is the KA there?

The PFC for the individual circuits is 22KA (11KA measured between L-N).

But the rating of the mCB`s are 10KA and I can`t find any rating on the main switch.

 

[marcshaw]

The main switch wont be a problem as long as its rated equal to or above the main fuse (amps), most main fuses in the electricityboard cutout would be rated above 20KA in industrial areas, and I would expect them to be rated at 33KA. I cant think of any MCB that is rated above 16KA. Not sure what to suggest really, I have never come up against such high readings. Is the place next door to the Sub Station!!!

 

[zsoltimatrix]

The main switch wont be a problem as long as its rated equal to or above the main fuse (amps), most main fuses in the electricityboard cutout would be rated above 20KA in industrial areas, and I would expect them to be rated at 33KA. I cant think of any MCB that is rated above 16KA. Not sure what to suggest really, I have never come up against such high readings. Is the place next door to the Sub Station!!!

Well, the factory has its own supply and this newly installed distribution board is next to the main distribution board. The isolators in there are 38KA rated.

 

[marcshaw]

I Think you should be fine, as i am sure you can rely on the main fuse for your KA breaking

 

[dichroic]

The breaking capacity rating (short-circuit capacity) of each automatic disconnection device needs to be adequate for the prospective short-circuit current and earth fault current at its point of installation. Alternatively, suitable back-up protection must be provided by another protective device, as required by Regulation 434.5.1.

A lower breaking capacity rating is permitted where so-called ‘back-up’ protection is provided. Back-up protection is where another protective device (or devices), having the necessary rated short-circuit breaking capacity, is installed on the supply side, and the characteristics of the protective devices are co-ordinated.

The co-ordination is required to be such that the energy let-through (I2t) of the device or devices does not exceed that which can be withstood, without damage, by the device or devices on the load side. Where back-up protection is required for a fault current protective device, the manufacturer of the device will normally provide information on the selection of suitable devices for back-up protection.

Hope this helps

Dichroic

 

[zsoltimatrix]

Well, I am a bit stuck on this one now....

Interesting is that I have just looked at the example certificate in On Site Guide, and there the example PFC is 16KA while the rating of the ring MCB is 6KA. So I gues it is allowed than as long as the main switch clears the PFC.

Thanks guys so far!

 

[dichroic]

As given in Regulation 434.3, there are circumstances where, for reasons of practicability or safety, it is permissible to omit a fault current protective device. Regulation 434.3 states that:


(i)a conductor connecting a generator, transformer, rectifier or an accumulator battery to the associated control panel where the protective device is placed in the panel

(ii)a circuit where disconnection could cause danger for the operation of the installation concerned, such as those quoted in Regulation 433.3.3

(iii)certain measuring circuits

(iv)at the origin of an installation where the distributor installs one or more devices providing fault current protection and agrees that such a device or devices afford(s) protection to the part of the installation between the origin and the main distribution point of the installation where further fault current protection is provided.

Provided that both the following conditions are simultaneously fulfilled:

(v)the wiring is carried out in such a way as to reduce the risk of fault current to a minimum (see item (ii) of Regulation 434.2.1, and

(vi)the wiring is installed in such a manner as to reduce to a minimum the risk of fire or danger to persons.




Best i can do to help.

Regards

Dichroic