instantaneous setting co-ordination on High voltage system

[SELEC]

Hi, Folks,

Inst. settings are very sensitive. Sometimes I try to avoid it for the main and branches. Per code, CEC or NEC, we are not allowed to disable them. So what I normally do is to set Inst on Main higher than the branch fault currents(Max.available), which is reduced by impedance in between. But sometimes on some high voltage systems, it is very hard to do the co-ordination study because the impedance makes little deference between fault currents on two levels of buses. Who have a better solutions?

thank you in advance.

 

[alehman]

This is a common problem. There's not much you can do except to use ANSI (C37.20) low voltage power circuit breaker switchgear which permits use of breakers without instantaneous trip.

Square D has a new line of 'high instantaneous' MCCB's which they advertise to help with this situation.

Both of these options of course increase the potential arc flash energy.

 

[SELEC]

It is a common problem. Actually our breakers are controlled by SR750 relays at upstream and downstream respectively. Altough I am trying to set downstream CB trip faster by lower current setting, and to set upstream CB slower by higher setting. Hopefully, this will work.

 

[dpc]

I don't think there is anything in the NEC that would require use of instantaneous tripping on a medium-voltage breaker or a LVPCB. If you are using an SR750, the instantaneous trip can easily be disabled.

Molded case circuit breakers are required (UL & NEMA) to have instantaneous tripping.

 

[alehman]

dpc,
I think you are correct for medium and high voltage - which is how the original question was worded, I see.

In the U.S., LVPCB's come in two versions -

UL 489: which has inst. (and no way to disable it) for use in boards constructed to UL 891 switchboard standards.

UL 1066/ANSI C37: which is not required to have inst. (and if equipped with inst., may have on 'off' setting) for use in boards constructed to UL 1558/ANSI low voltage power switchgear standards.

 

[davidbeach]

Actually LVPCBs only come in one version, the ANSI C37/UL 1066 version. Breakers which may be otherwise identical but listed under UL 489 are Insulated Case Circuit Breakers (ICCBs). It may be that the only difference is on the label, but by definition LVPCB is per the ANSI standard. Back when LVPCB and iron frame breaker were interchangeable terms, there was a very clear distinction between LVPCB and ICCB, but now you may have to look at the label to figure it out. You can buy ICCBs that don't include an instantaneous setting in the trip unit, but they do have a high set fixed instantaneous override. As long as the instantaneous override is above the available fault level, they can be treated as if they truly didn't have an instantaneous setting.

 

[alehman]

I think the definition is getting fuzzy with the newer plastic-enclosed breakers (e.g. MasterPact) that have some characteristics of both types and can be listed either way, depending on the trip unit characteristics. I would call those LVPCBs regardless of the listing, because they are constructed and tested to LVPCB standards. Perhaps that is incorrect.

To me, an ICCB is constructed and tested per UL 489. Typically they are not designed for switching duty or extensive field maintenance and do not have field-replacable contacts. Examples are Cutler-Hammer SPB or Square D SE. These seem to be being phased out by their manufacturers.

From IEEE 1015 -

Insulated-case circuit breaker (ICCB): A circuit breaker that is assembled as an integral unit in a supporting and enclosing housing of insulating material and with a stored energy mechanism.

Low-voltage power circuit breaker (LVPCB): A mechanical switching device capable of making, carrying and breaking currents under normal circuit conditions and also, making and carrying for a specified time and breaking currents under specified abnormal circuit conditions such as those for short-circuit. Rated 1000 V ac or below, or 3000V dc and below, but not including molded-case circuit breakers.

By these definitions, it would seem the newer plastic-housed breakers such as MasterPact could qualify as both.

 

[davidbeach]

alehman, I agree that the Sq D MasterPact and the Eaton Magnum DS get used both ways, and that the old days with the DS as the LVPCB and the SE or SPB as the ICCB made things much clearer. Both of those manufacturers designed those breakers to meet the requirements of both standards, but the standards do have enough differences that dual labeling probably won't happen. Voltage ratings being one example, where the UL breakers are rated for 240/480/600V nominal and the ANSI breakers are rated for 254/508/635V maximum.

For me, at least, while there are the two standards and they have different requirements, it makes sense to hold onto the older names, so that an LVPCB goes into ANSI switchgear and an ICCB goes into UL switchboards. I doubt the two are physically interchangeable, and until/unless they become interchangeable, it is useful to know which one is dealing with.