Education time :)

**Dylboy** · 29-Mar-22, 07:36 PM

Hello

Thanks to some encouragement I have decided to ask some questions as time goes of things I don't understand or feel I need more info on.

I would rather look dumb but know the answer than act smart and be wrong.

So to start:

I was taught that when we do the PSCC, we switch off the main breaker in the main DB in the house, and run the test. Let's say we get 2.8kA. So with that then the KA rating of the main switch we tested and then every circuit breaker in the DB must then have a KA rating of 3KA as minimum. So if we found a 2.5KA CB we would then note it to change for the CoC as needs to be 3KA.

Now I have been reading more and trying to apply logic but surly that PSCC test is testing the current rating of the supply cable to the transformer and that then the circuit breaker at the meter point or kiosk out side should only be the breaker that must be 3KA. ? Usually a 80A CB in domestic homes.

And that the circuit breakers in the DB would have to have their own PSCC done (by means of getting an ohm reading of Line and Neutral as it is a dead test and then doing Ohms law go get Amps. I.e 230 / 0.45= 511Amps thus the minimum of a 2.5KA CB is fine.

I would prefer to do a PSCC of the full circuit including the external loop as that is then the true resistance of all cables involved and transformer etc.

Hope this makes sense and sorry if looking dof hahah.

On a caviet I have booked for the unit standards course in April, so if all this is covered in detail there then that is great but if not would rather get a head start so to say on my questions.

The P1 and P2 course I did did not cover any of this, at least to the detail that I required to understand. The language used is not always easy to understand in the book and if you don't know what to look for it can be easily missed and confusion can happen.

Also I don't know many other wireman to ask in person and the ones I do know don't quite catch my drift or at least don't know the nitty gritty of formulas or reasons.

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**Alfred M** · 29-Mar-22, 10:21 PM

You have done the Ze (external supply) You need to do ZS (R1+R2) with all breakers energized on db and sub db's do a loop impedance (PSC on entire installation and calculate the short curcuit current from ZS loop impedance value if your instrument can only do loop impedance. My instrument can measure PSC as well on entire installation. Depending how close you are to the supply transformer the higher the PSC will be.If ka rating on ZS is higher than the ka rating on the curcuit breakers and rcd's you need to change them to higher ka rated devices.

**Alfred M** · 29-Mar-22, 10:34 PM

https://youtu.be/_twVvsGo81A
Informative video on ZE and ZS

**GCE** · 30-Mar-22, 05:42 AM

I am no expert in PSCC but my understanding on the subject is

Prospective Short Circuit Current is basically the possibility of the fault current occurring in a system network , both in the LV and MV networks

Each item along the way contributes to the reduction of the potential fault current. Each item can also create a risk of increased fault current in the case of a loose connection , theft on earth points etc.
PSCC is not an exact science , and can vary depending on factors outside of your control which is why people err on the side of caution when calculating.

On MV side it is often the case that it appears that cables are overdesigned for the amperage that they are carrying and this is mainly due to the fault currents that need to be taken into account.
If you had to take a supply of 130 Amps at 11KV in looking at cable size you would go with a 35sqmm cable.
You then need to calculate the potential fault current - To calculate the PSCC you need to know the maximum fault current that may occur and for what length of time it will be present before tripping . The trip times will be the settings on the MV switchgear .
Suddenly you end up with a 13 000KA potential fault level and your 35sqmm cable will be like a fuse wire - If you do further calculations you would need a 95sqmm cable to handle the fault.
On MV you will generally find that the fault level will determine cable sizes

On LV side
The transformer will have a rated fault current which will determine the size of circuit breaker - Using an 800KVA transformer it will generally have a fault level of around 25KA
In the old days it was only cables and fuses that could reduce fault levels but with the advent of electronics being fitted into circuit breakers things changed and circuit breakers can now also reduce fault level currents due to quicker response times and that they rupture the fault as close to the zero cross over point in the cycle .
From the 25KA circuit breaker you will run a cable of x meters to a LV distribution board or point - The cable will reduce the fault current further and can be calculated or derived from manufacturers tables and the DB may now end up with a fault current of 19KA on supply side .
You will more than likely go for a 25KA circuit breaker to be on the safe side , because maybe that cable was not manufactured 100% or the teams pulling it in find a slightly shorter route or the transformer winding impedance changes due to heat .
It is a Prospective or Potential or could be , might be short circuit current . It is like you are doing an estimate on what could happen if all the stars are in the same line , if one of them are out of line things could change quickly.

Coming to your original question ,which I got sidetracked on , with the advent of PSCC meters you now don't have to go find the transformer , measure the cable length and sizes and sit for hours punching calculations.

If you measure/calculate the PSCC at the supply side you would then calculate or measure further down as well .
In a commercial building you may have an incoming fault level of 50KA and as you move along you will end up with 5KA fault level - You would not stipulate that all circuit breakers must be 50KA as it would be an expensive build and so the time taken going through calculations is worth the effort.
Once you get to 5KA you will not bother trying to see if you can drop to 3KA as the difference in price is minor on the bigger picture and you would stick to 5KA minimal size through out.

In a domestic installation if I am going to toss between going 2,5Ka circuit breaker or 3Ka circuit breaker I would go 3Ka from a safety point - How often do you do a measurement and tomorrow test on the exact same point and the reading will be slightly different.
The municipality could change the 50sqmm cable in the street to 95sqmm and the whole calculation and readings will change

Most meters only operate up to 100Amp supplies and the reason behind it is due to circuit breakers that need to be taken into account - That is why if a board is cascaded you cannot take circuit breaker manufacturer X and just replace it with Y

**Dylboy** · 30-Mar-22, 10:14 AM

Morning

Thank you gents.

So yes I have now been working with Ze and Zs as also then you can do circuit design with regards to lengths, trip times and fault clearing times via adibitaic equation. So in the UK they give you max Zs values for a particular CB and then its trip time as well. So they seem leaps ahead of us.

I don't know where in Sans book or if this method is tought at the unit standards course.

Here we are told double the rated current to trip and also then given a table to follow of max Recc and then also a Lmax formula. So bascially 3 methods or ways to calculate a msx circuit length but non that I know takes in Ze or R1 + R2.

From further reading Sans, the double the rated current is to trip the MAIN protective device TO the installation. Thus meaning the breaker in the kiosk feeding the house. So that at least is now clear. So on the CoC when we do the PSCC at the main switch at the DB in the house and we test Ze, we make sure the breaker in the kiosk can handle that KA and also trip at double the rates current.

They don't ask for PFC for any other circuit I.e the Zs of individual circuits.

So yes from now on I will apply the Ze and Zs and do so appropriately in terms of which device, CB, must have that KA rating.

GCE, thank you for that reply as well! That is mostly how I understand it as well.

Not sure why we were told if the smaller CB was not rated to Ze KA then its a fail. So I am glad I have done this exersize.

Bascially a KA is needed for each circuit which includes the Ze and R1 + R2 and then to make sure that circuit is correctly KA protected.

Thanks guys!

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**Derlyn** · 30-Mar-22, 11:32 AM

Originally Posted by Dylboy

Morning

From further reading Sans, the double the rated current is to trip the MAIN protective device TO the installation. Thus meaning the breaker in the kiosk feeding the house. So that at least is now clear. So on the CoC when we do the PSCC at the main switch at the DB in the house and we test Ze, we make sure the breaker in the kiosk can handle that KA and also trip at double the rates current.

Only problem is that we do not have access to the kiosk and don't really know what breaker is in the kiosk supplying the installation.
Here I'm talking domestic.

Here in our valley, more and more kiosks are being installed on top of the pole.

**Dylboy** · 30-Mar-22, 11:35 AM

Hello !

Yes ya that is true, but i mean more the CB where the Meter is sorry, unless not every bundling is like that but the homes I have seen have the CB by the meter. So when I say kiosk I apologise and mean the meter box or meter point then.

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**Derlyn** · 30-Mar-22, 02:01 PM

No problem. Here there's no breaker at the meter. From the council kiosk directly to the meter and from the meter directly to the db.

We also do not use isolators for the main switch, but circuit breakers. The reason for that is the council wants supply to trip on installation first instead of in their kiosk in the event of a serious short or fault on the installation. On sub db's we use isolators or earth leakage for the mains, but on the main db we use a circuit breaker.

**Dylboy** · 30-Mar-22, 02:34 PM

Originally Posted by Derlyn

No problem. Here there's no breaker at the meter. From the council kiosk directly to the meter and from the meter directly to the db.

We also do not use isolators for the main switch, but circuit breakers. The reason for that is the council wants supply to trip on installation first instead of in their kiosk in the event of a serious short or fault on the installation. On sub db's we use isolators or earth leakage for the mains, but on the main db we use a circuit breaker.

Oh intresting, yes ya, we also use a CB as main switch, generally 60A as the CB in MP is 80A, for discrimination.

So in your case, not having the CB in the MP, doing the Ze for the purpose of main switch in my eyes now is useless, if there is a short on that feed cabke to the main switch it will take out city power stuff.

So your KA rating is determined by a short bascially from output of your mainswtich. Example, the phase hitting the Neutral.
In essence testing on the output of your main breaker with the rest of the individual CBs in the open position to then get the KA for the main switch only. Thus being the Ze.

Then would have to do a KA rating for each CB independently to size that breaker occodingly. Ideally with a MFT and do a Zs loop test with no trip.

Or using the main breaker Ze and measuring R1 + R2 and adding them up.

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