TRUE OR FALSE CURRENT LIMITING #2 2

[tulum]

I have already asked the question to Schneider Product Specialists in the states and have not received much help.


Here is my situation,

I have a system with a transformer that provides approximately 18-20kA of fault current. What is proposed is to have a Cutler Hammer ND main breaker on the secondary that is good for 25kA - This is fine.

The problem is that on the load side of the main breaker it is proposed to put a couple Squared combination starters size 2, size 4, and size 5.

Note - the components are Squared but the manufacturing has been farmed out to a local panel builder - i.e. not in squared enclosures.

Here is a breakdown:

Size 2: Mag-Gard (FAL3605016MV) 50A MCP, with Squared Size 2 starter (8536SDO1V02H20S) and O/L's

Size 4: Mag-Gard (KAL3625025MV) 250A MCP, with Squared Size 4 starter (8536SFO1V02H20S) and O/L's

Size 5: Mag-Gard (LAL3640032MV) 400A MCP, with Squared Size 5 starter (856SGO1V02H20S) and O/L's

The MPC's do not come with an interrupting rating alone - as per UL requirements. However, they do have come with a combination rating when used in combination with appropriate motor starters.

The sticker ratings that were shipped loose with the above are all 10kA. However, these ratings all seemed to be based on being tested within certain Squared enclosures as a combination (as per UL 508), i.e. in a squareD standard enclosure a size 4 is 22kA and in a Nema 7-9 enclosure it is 10kA. This seems to be more of an indication of the amount circuit current a enclosure can contain without deforming, rather than component classification.

My question is:

I want to know if the above starter COMPONENTS are suitable for use on a system that can supply 20kA fault current, i.e. enclosure not accounted for. My gut is this is a bad setup all together, and the spec should have included fused disconnects.


Regards,
TULUM

 

[buzzp]

Not sure this will answer your question but it should help. Motor overload devices are not meant to interrupt fault currents, neither are motor contactors or starters. They are for overload conditions not short circuit conditons. This would mean, the starter interrupting current is only applicable up to LRA of the motor since this is the highest current it has to interrupt. Anything higher, the breaker should be clearing. This is from the perspective of UL508. This is another reason overloads have some definite time associated with them before they trip, to allow the short circuit protection a chance to clear the fault.

 

[alehman]

Keep in mind that regardless of whether there is an applicable series rating available, the NEC doesn't permit use of series ratings where current from the motors downstream exceeds 1% of the AIC rating of the lower rated device (combination starter in your case).

From the Digest section on MCP's: "Interrupting ratings are established for these UL Recognized Components only when they are used in combination with motor starters with properly sized overload relays and contactors." Therefore the combination starter assembly would have to be part of the series rating. I can't recall ever seeing a series rated combination starter with MCPs, fused or otherwise.

Doesn't help much.

 

[tulum]

buzzp and alehman,

Thanks for the comments, I will respond best I can, I do not have as much experience as the two of you seem to have - past posts.

buzzp:

I believe what you are saying about LRA to be true, however, the starter and O/L's come with a sticker that says "this component can only be used on a system that is capable of delivering not more than 10,000A RMS SYM.".This makes me believe it is some sort of withstand rating?

alehman:

This is exactly how they are listed in the squared digest - as combination ratings. My big problem is the rating they give changes depending on what size/type of enclosure you buy... I am not buying there enclosure.


If I change out the Mag-gards and put in thermal magnetic breakers, could I achieve I higher rating?

Regards,
TULUM

 

[buzzp]

Tulum,
I am not sure what they are actually telling you with this rating. But since the SYM is on the end, I would believe this rating has nothing to do with fault currents.
For your application (you have short circuit protection from an upstream breaker), you only need to be concerned with the locker rotor amps as the starter is only going to have to interrupt this as a maximum. Anything above should be coordinated to allow the short circuit protection to handle.

As I said, I do know for a fact that a starter (contactor with OLs) can NOT interrupt fault current has approved by UL. I have designed some electronic OLs in the past and this was something UL watches carefully. We have to have a minimum trip time to allow the short circuit protection to clear the fault.

 

[alehman]

Sorry I misunderstood the point of your question. Unfortunately I don't think there's an easy answer. The assembly without the enclosure may not be UL listed and therefore probably has not been tested by SqD.

Changing to a fused switch or breaker might make the inspector more comfortable, but doesn't fix the fundamental problem. I think your panel builder is going to have to consult SqD engineering an decide if they can UL label the completed product.

Not much help.

 

[tulum]

Thanks for the help guys.

I have looked at both AB and SquareD and the ratings are the same. If it is a breaker or a MPC the publish a listed combination as tested within a certain type of enclosure. If it is a starter combination with a fused disconnect the rating is for the cmbination independent of the enclosure. This just boils down to type1 and type2 protection.

How I will get a suitable rating in a panelbuilders box without going type 2 is another story!

Thanks agian folks.

Regards,
TULUM