Panel Fault Level

Fault bracing / interrupting capacities for electrical equipment are based on the levels at the incoming line terminals of that equipment. If you have already certified that it is 45kA, it is 45kA.

Thanks for the reply. The Panel-1 incomer will be at 65kA. can the outgoing feeder (Feeder-1) be at 45 kA?

Prashanth

Read #1↑ again.

... and repeat until understood.

You are doing stage 2 before stage 1.

Find out what the PSCC is. This is the current that could be delivered to your panel from your transformer. A 1MVA transformer is usually around 30kA. This will have dropped a little by the time you get to the panel.

Your MCCBs, ACBs or whatever you select must be able to withstand this current. If the board is a fault rated board then you will need to ensure the busbars are rated above your potential fault current.

There all sorts of things you need to consider from arc flash, discrimination, protection settings together with the need to employ a reputable panel builder.

The power you are dealing with, about 2.5MVA, is large enough to cause a serious explosion. You need professional help.

Quote "Thanks for the reply. The Panel-1 incomer will be at 65kA. can the outgoing feeder (Feeder-1) be at 45 kA?"

Prashanth

If the fault current at panel 1 is 65 KA, then every thing in the panel must be rated at 65 KA. The fault at the breaker for feeder 1 is 65 KA so it must be rated at 65 KA. Now the fault current at panel 2 will be less so you may have a different rating at that point.

Thanks Sir. Good reply.

Prashanth

In this case all devices in panel 1 to be rated for 65 kA and in panel 2 all devices to be rated for 45 kA. Please understand if the fault occurs at the outgoing terminal of the outgoing Mccb/ ACB,the fault current will be almost same with that at the incoming of the panel. Therefore , for any panel the incoming and outgoing devices like ACB / MCCB should have the same short circuit rating

Thanks Sir. Good reply. My doubt is clarified

Prashanth

yes correct

If The incomer ACB is 3200 A and the outgoings MCCBs linked with the same Bus bar in same enclosure it must be in a same short circuit capacity values.

Eng.Ihsan

Fault levels at various points in the distribution system should be calculated for various kinds of faults and loads taking into account the fault level at the point of connection. Motors too would contribute. Finally coordination of protective devices,settings of OC/SC/EF relays too should be studied.

"with 1 incomer and few outgoings"

"The incomer ACB is 3200 A, 65 kA"

"One of the outgoings (say, Feeder-1) of Panel-1 is connected to Panel-2, with MCCB as incomer. This incomer of panel-2 (as per our calculations) is at 45kA."

"So can we have Feeder -1 at 45 kA instead of 65 kA ?"

What is the kA of the "few outgoings"? Add the 45kA and the kA of the other "few outgoings". That will tell you what you need for "Feeder 1"

Simple addition. What's the problem?

I do not understand as to what you are trying to inform others , but your suggestion is misleading.

"but your suggestion is misleading."

How is it misleading?

Feeder 1 is rated for 65kA. It feeds Panel 2 with 45kA PLUS other "few outgoings". The OP is asking if they can have Feeder 1 at 45kA. The answer is Feeder 1 needs to be at 45kA PLUS the other "few outgoings". What is the difficulty in understanding that?

It's misleading, you don't add up fault levels. As said before fault levels are based on calculations of source and system configurations In answer to the OP. ACB's are generally not current limiting, so the 65kA at the incomer will also be seen at all the feeders. Hence the need for fully rated devices. If you had a current limiting device as the incomer, then you could technically have used reduced ratings on the feeders, based on the let through data. But as a note, if you are in an IEC country, then you should be working to IEC61439-2 and it requires your assembly to have design verification carried out, followed by routine verification on the production assemblies Which standard are you working to?

this suggestion is well mixed up. The 65kA is the maximum fault current occuring from a short circuit. There is nothing to add to this. This is all of the energy from the secondary of that transformer coming together.

Although the fault will drop slightly through the board/MCC we are looking at the impedance caused by the busbars. Its virtually nothing so a fault at the bars will still be in the order of 60 to 65kA. This is a big fault.

Every component that might see this fault such as one of the outgoing MCCBs must be rated to this figure. You might get a dead short on the outgoing side of the MCCB feeder.

If panel 2 is some distance away and a length of cable is required the circuit impedance will have increased and the current will drop. It is possible it will have dropped to 45kA by panel 2. Panel 2 could then be less onerous than panel 1.

Calcs still required to prove all of this though.

You really need to do the fault study to find out what values to use. If the incoming breaker will see 65kA on a bolted fault across the busbars then is must be rated to at least 65kA.

A panel of this type would need to be fault rated and that applies to the bus bars and the way they are braced together with all outgoing circuits.

If you use one of the outgoing ways of the panel to feed a second panel through a length of cable the fault level will have dropped at the second panel. Lets say the incomer to the second panel is 20m away then its likely that incomer could be rated lower at 45kA.

These are just words though - you need to determine the fault current at the different parts of the installation and ensure your equipment is rated at least equal to it.

I agree with silverfox. All the discussion has been based on the interrupt ratings of the breakers that the OP described (65kA and 45kA). However, no information on actual fault levels was provided! Until a fault study is done, the breakers may be overrated or overdutied. The breakers were probably specified based on "rules of thumb" and the kVA and % impedance of the supply transformer (if at all).

On short circuit calculation, and on discrimination / coordination / cascading between LV circuit breakers, you will find many reference documents on

http://www2.schneider-electric.com/sites/corporate/en/customers/designers/designers.page

http://www.electrical-installation.org/enwiki/3-phase_short-circuit_current_(Isc)_at_any_point_within_a_LV_installation