UL1077 MCB AND FUSE HOLDER VS MCB UL489

[cipoteviejo]

Hello, i am doing the design of 8 distribution panelboards and this will be connected to the output of 8 UPS`s. In the design, i used OCPD UL489 rated for 20A to 30A for branch circuits protection, but all existent panelboards in the refinery always uses OCPD UL1077 combinated with fuses. I am not sure that it be a good practice.


I have understood that NEC accept two types of OCPD (FUSE and CIRCUIT BREAKER) in fact, it is not apropiated to use UL1077 to protect branch circuits




Is there some restriction for using only UL489 Miniature Circuit Breaker without combin it with fuse?


I am worried because i need to build the panelboards without to commit errors.
I am worried because the customer likely could not accept the panelboards once built (with MCB only without fuses)


Excuse me for my english.

 

[petersonra]

A lot of these kind of things are more spec driven than anything else.

Not all fuses are suitable for use on branch circuits, just as not all CBs are.

The supplementary type CBs and fuses are intended to provide protection to a device as opposed to the circuit itself.

In general, I don't see a problem with having a fuse to protect the branch circuit and a supplementary protector downstream being used as a disconnecting means.

 

[cipoteviejo]

Please, see attached file.

The problem is that the both (MCB and fuse) are inside the panelboard. First the
MCB y the fuse is downstream, but inside the same panelboard.


In this case, I see that the conductor between the MCB(UL1077) and the FUSE is protected by UL1077 MCB, but this is not permmissed. On the other hand, the conductor between the MCB(UL1077) and BUSBARS is only protected by the MCCB upstream only agains shortcircuit (not overload)


The conductor between fuse and the load ,it is the branch circuit (conductor going outside panelboard) will be protected by the fuse. I dont see worth added using additionaly MCB (UL1077) upstream FUSE.

 

[jim dungar]

If the conductor is sized equal to the one feeding the UL1077 MCB, then the MCB is simply acting as a switch, so it is not a concern.
You need to look at how the conditions in NEC (2008) section 240.21(B) might apply to your panel. If this section is used correctly, I see nothing inherently wrong with the panel in your picture.

 

[cipoteviejo]

Jim Dungan,


I agree with you about the conductor between the MCB and the FUSE is protected but no by the apropiated OCPD, the MCB(UL1077) is not designed for this application in distribution panelboard but rather is designed for use as supplementary protection.


Please, see the attached file.

1) Is it ok to use MCB (UL489) alone as OCPD for branch circuit? I think, Yes.

2) Is it ok to use MCB (UL489) combinated with FUSE as OCPD for branch circuit? I think, Yes.
3) 1) Is it ok to use MCB (UL1077) alone as OCPD for branch circuit? I think, No. because MCB UL1077 is not for this application.
4) Is it ok to use MCB (UL1077) combinated with FUSE as OCPD for branch circuit? I think, No. because MCB UL1077 is not for this application.
5)Is it ok to use FUSE as OCPD for branch circuit? I think, Yes if the fuse is UL248.

what do you think about it?

 

[jim dungar]

4) Is it ok to use MCB (UL1077) combinated with FUSE as OCPD for branch circuit? I think, No. because MCB UL1077 is not for this application.

If the fuse is properly sized to protect the conductor, then the MCB is being used as a supplemental device.

 

[petersonra]

I also see nothing inherently wrong with this setup.

I don't see any real benefit to having the CBs in the circuit at all.

I would be inclined to just pull the fuse over opening the CB if I needed to deenergize the circuit.

To get at the CB, you have already opened the cabinet up anyway, so all the precautions that are necessary to do that are in place so what benefit is there to having the CB in the first place?

Personally, I would have designed this kind of thing using a 3 phase panelboard with a locking means on each CB. If I needed fuses for some reason, they would be added downstream, but I just can't see a reason to make something like this when a standard panelboard would handle it quite well.

Incidentally, QOU CBs are DIN rail mountable and BC rated. And usually cost less then these type of supplemental protectors. They only go down to 10A though.

 

[jdsmith]

I don't see any real benefit to having the CBs in the circuit at all.

I would be inclined to just pull the fuse over opening the CB if I needed to deenergize the circuit.

...Personally, I would have designed this kind of thing using a 3 phase panelboard with a locking means on each CB. If I needed fuses for some reason, they would be added downstream, but I just can't see a reason to make something like this when a standard panelboard would handle it quite well.

This panelboard is on the load side of a UPS - to minimize voltage dips during faults it is desirable to clear the fault as fast as possible, which means using fast acting fuses. Fused panelboards with fast acting class CC, RK1, J, or T fuses are typical for industrial UPS power distribution. The only reason the circuit breaker is ahead of the fuse is because those fuse holders are not labeled for service as a load-break device. The manufacturer's datasheet for the fuse holder is either moot on the topic of breaking loads, or it will specifically state that it is a non-load break device. The circuit breaker is not used for protection at all, only as a load break switch. Some manufacturers of similar panelboards in the US (Ametek, Filnor, VSI, Kinney) use molded case switches instead of circuit breakers. A molded case switch is a UL 489 device that does not have an overload element. A molded case switch still does contain a very high instantaneous (magnetic only) trip element because UL 489 require the molded case switch to protect itself from short circuits - meaning the switch has to open rather than staying closed and blowing up. To reduce costs the manufacturers I mentioned above buy standard bolt-on panelboard "guts" (like Square D NQOD or similar), equip the branch circuit positions with molded case switches, then run a short length of wire from the molded case switch to the fingersafe pullout fuse holder.

 

[cipoteviejo]

jdsmith, petersonra, jim dungar


there is some thing that we have not talked, most them (branch circuits) will be 3 phase. This branch circuits will supply power to 3 phase PDU`s (Bank 1 (L1-L2), Bank 2 (L2-L3),Bank 3 (L3-L1).


Is it corrrect that during an short circuit LG (Line to Ground) that other phases
do not break?.


Most of load are "rack- equipment" that uses conector IEC C13-C14 which catch energy from two phases.


What is your oppinion taking this into account?

 

[jdsmith]

jdsmith, petersonra, jim dungar


there is some thing that we have not talked, most them (branch circuits) will be 3 phase. This branch circuits will supply power to 3 phase PDU`s (Bank 1 (L1-L2), Bank 2 (L2-L3),Bank 3 (L3-L1).

Is it corrrect that during an short circuit LG (Line to Ground) that other phases
do not break?.

Most of load are "rack- equipment" that uses conector IEC C13-C14 which catch energy from two phases.

What is your oppinion taking this into account?

It doesn't matter. You are correct that for a L-G fault only one fuse would blow, but in many cases with single phase 208V equipment connected L-L you'll have a L-L-G fault and the fuses on both affected phases will blow.

 

[petersonra]

This panelboard is on the load side of a UPS - to minimize voltage dips during faults it is desirable to clear the fault as fast as possible, which means using fast acting fuses. Fused panelboards with fast acting class CC, RK1, J, or T fuses are typical for industrial UPS power distribution. The only reason the circuit breaker is ahead of the fuse is because those fuse holders are not labeled for service as a load-break device. The manufacturer's datasheet for the fuse holder is either moot on the topic of breaking loads, or it will specifically state that it is a non-load break device. The circuit breaker is not used for protection at all, only as a load break switch.

I had not considered the fuse would be a fas blow unit? Wouldn't they be suspectable to tripping on powerup?

I am not going to use the CB or the fuse to shut off power to downstream devices. I am going to go shut them down first anyway. There should not be a load on the fuse or the Cb when it is opened.

 

[jdsmith]

I had not considered the fuse would be a fas blow unit? Wouldn't they be suspectable to tripping on powerup?

I am not going to use the CB or the fuse to shut off power to downstream devices. I am going to go shut them down first anyway. There should not be a load on the fuse or the Cb when it is opened.

Typically the equipment connected to UPS's does not have significant inrush currents so fast acting fuses will not blow when powering up equipment.

I understand your operational decision to not open up the fuse holder or breaker until the load is off of the circuit - I do the same thing. The issue at hand is that these fusible panels are built and sold as products and the builder is concerned with Listing of the panel board as an assembly, which requires a disconnect for each branch circuit to be load break rated.