Fault Current protection

[psa807]

Can someone help me with this scenario?
I have a facility panelboard with a certain KA interupting rating. I then change out the transformer feeding the panelboard to a larger unit which increases the available fault current beyond what the panelboard is rated for. If I put a single breaker with sufficient interupting capability in series with the panelboard, is this acceptable? And my main question, do I have to coordinate the trip curves/time delays between the new breaker and the downstream panelboard main breaker, or does the new breaker with the proper KA rating meet protection requirements regarless? Thank you!

 

[charlie]

Re: Fault Current protection

Question 1, no, a series setup has to be tested in accordance with 240.86 so that it has a series rating. The series rating comes from the testing laboratory like UL.

Question 2, see the answer to question one. It is not permitted to engineer a series combination. A proposal has been made to permit an engineered series combination but it has not passed yet and may not pass the Code Making Panel.

 

[BAHTAH]

Re: Fault Current protection

Manufactures publish their series rating combinations. You may be able to purchase an enclosed breaker to place ahead of your panel that meets the series rating combination rating however this could all be effected by what motor loads you have connected to the panel and the series rating limitation of 240.86(B). Another option may be the use of Cable Limiters which provide short-circuit response but not overload protection.

 

[brian john]

Re: Fault Current protection

As for your second question, Do you have to coordinate these CB's New to old, Well only if you want to avoid nusiance tripping of the new hiher AIC main (If the installation is approved).

 

[charlie]

Re: Fault Current protection

Grant, that would still be an engineered combination and is not permitted!

Brian, please look again at my answer to question 2, ya can't do it!

The reason for the problem is that the current limiting device in front of the old equipment will try to open up at the same time as the existing overcurrent device. The dynamic impedance introduced by the older OC device will lower the fault current so the first OC device will no longer be current limiting (opening in the first 1/4 cycle) and the second (older) OC device could be in danger of failure. In other words, let's just paint it OD and put a pin in it (make a grenade for those who haven't been in the military)!

The bottom line is that you could be building a bomb, even if it is approved!

 

[brian john]

Re: Fault Current protection

Charlie:

Not knowing the age of the equipment, manufacture or design I was not commenting on item 1, only on the fact that in any installation proper coordination of overcurrent protection devices should be a standard part od design.

 

[charlie]

Re: Fault Current protection

Sorry Brian, it sounded to me like he could install this if he got it approved.

 

[templdl]

Re: Fault Current protection

Brian, the aic of the device doesn't have anything to do with nuisance tripping. The magnetic or instantaneous calibration of the devices, if 10, 22, 25, 30, 100 kaic are all the same. It's the moving and stationary contact design which allows the breaker to open up the contacts fast enough to clear the fault before the breaker comes unglued.
It is important to coordinate the kaic rating of the breakers as the others have pointed out and the only way to do this is by the manufacturers series ratings. These are real tests which are conducted that assure that the upstream OCPD has the ability to open fast enough to limit the current to prevent the downstream OCPD for blowing apart.
Please note that this does not necessarily mean that the upstream device is actually UL listed as current limiting. All breakers are current limiting to some degree, but some are more current limiting than others to a point where there are those that can be actually listed as current limiting.
Bottom line? As the other have said, the breakers in the system MUST BE listed together otherwise all must be 100% rated for the available fault current.
Also, depending upon the complexity and size of the distribution system a coordination study done by a qualified person can determine what aic is available at different locations in the distribution system. This may or may not be of benefit.

 

[ron]

Re: Fault Current protection

Good points. Just to clarify a term that is being used in different ways.
To "coordinate" devices means to ensure that the down stream device will open prior to an upstream device, if it can be adjusted or selected to do so.

 

[templdl]

Re: Fault Current protection

Ron, It's good that you emphisized that point. Overload or I2t sensing is fairly easy to do but short circuit depends upon big time luck. If there is a bolted fault on one of the branch circuits which draws basically the entire available fault current from the service entrance then all of the OCPDs that feed into that fault will see that current. If that current is within the magnetic trip range of any or all of those breakers it is pot luck which one will trip. It's not uncommon for an upstream or main breaker to trip.