utility company fault rating 10

[stevemechanical]

what happens or what can be done if the utility company current fault rating (13,000) is greater than the current fault rating of the equipment installed (10,000)?

 

[davidbeach]

Install different equipment. Or add an inductor to limit the current.

 

[Zogzog]

The simple answer to your question is KABOOM! The protective device must be able to handle the available fault current in the system.

 

[dpc]

This is an NEC violation and an unsafe operating condition. You need some additional impedance on the service - a smaller utility transformer, a transformer with a higher impedance, or as mentioned, a current-limiting reactor.

You might also check to see if you can get a series rating by installing a main fuse that is a listed combination with your downstream breaker.

 

[RalphChristie]

Or you can contact your local utility company. Supplied fault-ratings provided are worst case scenario's and include many times possible future growth (might want to implement a bigger trsf, etc) If you are lucky you might negotiate a mutual agreement.

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[rbulsara]

What size and type of equipment you have? ampere, voltage, breaker or fuse etc..?

More information may help formulate more practical solutions. Plus where are you and the site located?

 

[Senselessticker]

stevedantonio,

What equipment exactly is rated at 10k? If it is the protective equipment just downstream of the utility, then you have a problem. If the 10k rating is for protective equipment farther downstream (breakers in a lighting or receptalce panel for example), then you may be okay. But you would need to perform a fault calc to be sure.

Give us some more details and I'm sure folks here can help.

 

[waross]

If the fault current is rated at the transformer terminals, calculate the impedance of the service conductors to your main breaker. This may add enough impedance to limit the current to 10,000.
respectfully

 

[stevemechanical]

the utility company's 3 ph bolted fault at the point-of-service is 13,737 amps/ph. on 120/208volt sys.
After the point-of-service we have the end box, 800A trans s cabinet (50 KAIC), 800A s/s(200KAIC) then the mdp at 10KAIC

thank you

 

[rbulsara]

Please be more clearer..what is 800A trans s? and 800A s/s??

What and where is the main disconnect device (fuse or circuit breakers) in above two?

What type is MDP? I assume breakers is it is 10000 AIC.

If the main device is the fusible switch, you may be able to find out (from manufactures of the MDP) if the breakres in them are series rated (UL listed) with fuses in the main device (should be Class L) for higher AIC rating.

As others mentioned if you take in to account the cables between POS to the MDP it may be sufficient to drop SC current available at MDP but it needs to be calcaulated.

Sounds like it's time to involve a professional electrical engineer in this case or replace the MDP with a 22KAIC rated one.

 

[stevemechanical]

i'm sorry for the abbrev. i intended to write
800 amp current transformer and 800 amp service switch.

 

[EPEngineer]

stevedantonio,
I believe there is a mistake in your 2nd last post (200kAIC for service switch). If is 20kAIC, then it seems OK for 13.7kA fault level. What is the cable size and no. of cables/phase from Service switch to 10kA MDP (I think it is Main Distribution Panel). If the length is more than (roughly) 50 meters, I think you won't be in problem.
Expert comments are welcome.
SMA

 

[stevemechanical]

the service switch is a siemens heavy duty, type 1 (indoor)it is rated at 200,000 AIC with class L & T fuses.
the new electric service is 4-4/0 MCM CU UG service to P/L.
the main distribution panel is 20 ft away (in the same room)
thanks

 

[stevemechanical]

from service switch to mdp there are (2) separate metal conduits with (4) 500 mcm in each.
thanks

 

[sitl]

1. The switch breaking (making?) capacity is 200kA only if there is upstream protection device (breaker or fuse), otherwise is probably less than 10kA.

2. Problem may be not only the equipment in MDP, but also in subsequent switchboards if their breaking capacity is low (6kA or less). MDP is quite close to end box. In order to be sure, I would think about cascading the equipment instead changing the equipment in MDP. Cascading provides circuit breakers placed downstream of a limiting circuit breaker with an enhanced breaking capacity. The limiting circuit-breaker helps the circuit breaker placed downstream by limiting high short-circuit currents. Cascading makes it possible to use a circuit breaker with a breaking capacity lower than the short circuit current calculated at its installation point.
This means that I would install limiting circuit breaker instead of service switch. Important: cascading should be implemented only by using circuit breakers predefined tables made by their manufacturer. You should be able to find them in Schneider Electric, Siemens, ABB or GE site or in their catalogs.

 

[davidbeach]

sitl, Please, Please, do not use the term "cascade." What is defined in the tables you reference is referred to as "series rating", not cascade rating. The cascading technique was something else that is no longer acceptable because it didn't work.

 

[sitl]

to davidbeach:

Sorry David, there must be some misunderstanding.
I think that you are wrong, since the cascading was officially implemented in IEC 60947-2 and NFC 15-100, &434.3.1 (with that specific term). If you open catalogs from this year from, for example ''Schneider Electric'', you will find the same term ''cascading'' and the same tables as in previous documentation.

 

[davidbeach]

OK, cascade may be an IEC term, but in the ANSI world what you are describing is series rating and cascade is something else (used up-over-and-down calculations from current limiting fuse and breaker let-through graphs).

 

[JJHorak]

You might revisit how they came up with 13,737A. There was likely 2 significant digits of tranformer impedance. Maybe if they included only tiny bit of neglected impedance fault duty would go down.

Side issue: Under what conditions does one use 10% over voltage to calculate fault duty?