Breaker Adjustment

[mbrooke]

Why do some breakers let you turn off the instantaneous function? What happens if its turned off?

 

[paulengr]

Only the branch circuit overcurrent protection needs or should have instantaneous turned on most of the time.

As a general question the normal inverse time curve still applies. It just takes longer to trip. In large older breakers it might take 2-4 seconds, an eternity in breaker time.

In terms of faults we basically have 4 kinds. First we have overloads. These can eventually cause thermal damage but we have time (seconds) to respond and we don’t want to nuisance trip motors and transformers. Second is ground faults. These can be minor (GFI range) or major (treat like shorts which they are). Third is dead shorts and a close fourth is arcing shorts. These are highly damaging and we need to trip as quickly as possible when detected.

But if you have a short circuit, which one trips first, the branch or the feeder? Do you want to remove the fault at the branch or black out the whole building because of a single light fixture? Scaling this up imagine what happens at the utility level. A fairly famous case shut down most of the Northeast US stretching from Ohio to Maine because of a coordination issue.

Coordination is recommended under NEC. In some Codes such as hospitals it is REQUIRED. Coordination means the breaker closest to the fault and ONLY that breaker trips. Miscoordination means it happens out of order.

That being said there are reasons it usually works out. First we have dynamic resistance. Series breakers tend to just naturally coordinate anyway as long as they are different in size enough. You can’t rely on this and only tested combinations are actually allowed if coordination is required. With fuses there are generic charts for this. So generally speaking even though it’s not intentional a 200 A main won’t trip before the 15-20 A branch breakers in a distribution panel, even though these don’t allow disabling instantaneous.

Second is these days sometimes we do this intentionally. It is becoming common practice for maintenance groups to install a “maintenance switch” on feeder breakers. Quite often this switch just turns instantaneous on and off. With it on it miscoordinates but trips much faster if an arc flash happens. With it off the normal thing happens.

Also there is zone interlocking. It’s great in concept but I’ve never actually seen it installed anywhere. Downstream breakers interlock to upstream ones via a 2 wire bus so that all breakers can trip instantaneously.

 

[mbrooke]

Why is there instantaneous in the first place when it seems like thermal curves coordinate but magnetic ones can overlap?

 

[paulengr]

Why is there instantaneous in the first place when it seems like thermal curves coordinate but magnetic ones can overlap?

In older breakers you can buy just the trip functions you needed. Like just LS or LSG instead of say LSIG. But in modern microprocessor breakers it’s all the same cost so no reason not to offer one trip unit that does everything.

 

[paulengr]

In older breakers you can buy just the trip functions you needed. Like just LS or LSG instead of say LSIG. But in modern microprocessor breakers it’s all the same cost so no reason not to offer one trip unit that does everything.

Also most short circuit faults are at the loads so if only branch breakers do instantaneous, there isn’t an issue.

I forgot a third case for doing it. If the current setting is higher it still coordinates. So if short circuit is say 3-10 kA at the branch but 15 kA at the feeder it’s easy to coordinate them, but few installers have access to full power system studies. And dynamic resistance is almost a crap shoot except on new installs, all the same brand.

 

[mbrooke]

Yes and Yes, but none of this explains why instantaneous trip is added to feeder breakers or why it can be over ridden.

 

[paulengr]

Yes and Yes, but none of this explains why instantaneous trip is added to feeder breakers or why it can be over ridden.

You need to be able to turn it off in feeders. That’s why it can be turned off. It is there if you need it. But it’s a breaker. If it for instance fed a large transformer, then it’s not a feeder breaker and it needs instantaneous.

I’ve seen older ones without it so it is sometimes an option.

 

[mbrooke]

But why do some (most) not give you the option of disabling it?

 

[paulengr]

But why do some (most) not give you the option of disabling it?

It’s a hardware feature in many. One of the issues with microprocessor based trip units is boot time. If you have an external relay running off DC that’s one thing because the relays can boot up before the breaker closes. But if it’s self powered off the power bus as the integrated units are if the breaker closes into a dead short you need something not software based as a fail safe. As discussed previously we can wait when it’s a thermal trip but not short circuits.

 

[wbdvt]

Sometimes the instantaneous is turned off to allow for coordination with downstream devices. In this case, the breakers short time withstand rating is utilized.

 

[mbrooke]

I hear you, but I was told the instantaneous was/is necessary according to some UL standard.

 

[paulengr]

I hear you, but I was told the instantaneous was/is necessary according to some UL standard.

For a specific type of molded case breaker, yes. That’s UL 489. For thermal/magnetic breakers. And there is a limit on how high (10x). And the long term trip cannot be adjustable and you have to pick trip range B, C, or D.

But what we are talking about are what we’re formerly known as “ANSI” or “IEEE” breakers. Those fall under UL 1066. Switchgear standards are UL 891 and 1558 but there is some crossover between them, assuming you stick with UL standards instead of the older ANSI standards. Modern breakers tend to be the SAME breaker for both standards just with different trip units and in 1066 configurations it is typically bolted onto a draw out frame. This is when we get into insulated case, MV, and “power circuit breakers”. Often in switchgear the circuit breaker is just that...a high interrupting current switch...where the trip unit is a completely separate device.

As you get into distribution systems there are different OSHA regulations, NESC replaces NEC, and instead of NRTLs you write specs and require/read test reports.

 

[ron]

Generally you will only find the ability to turn off the instantaneous on UL 1066 Low Voltage Power Circuit Breakers installed in UL 1558 Switchgear because it has a 30 cycle short circuit rating.

A breaker feeding equipment that only has a 3 cycle short circuit rating, like UL 891 Switchboard, needs the instantaneous turned on to protect it past 3 electrical cycles.

 

[mbrooke]

Generally you will only find the ability to turn off the instantaneous on UL 1066 Low Voltage Power Circuit Breakers installed in UL 1558 Switchgear because it has a 30 cycle short circuit rating.

A breaker feeding equipment that only has a 3 cycle short circuit rating, like UL 891 Switchboard, needs the instantaneous turned on to protect it past 3 electrical cycles.

This is the answer I was looking for! Though couldn't a fault last longer than 30 cycles on thermal on lower fault currents?

Any idea if old switchboards and load centers were rated 30 cycles as a lot of them either had no mag trip or it was well over 20X?

 

[steve66]

I assume a fault over 30 cycles would put you into the short time rating.

 

[mbrooke]

I assume a fault over 30 cycles would put you into the short time rating.

Though it might not, depends on the run and source Z.

 

[jim dungar]

A breaker feeding equipment that only has a 3 cycle short circuit rating, like UL 891 Switchboard, needs the instantaneous turned on to protect it past 3 electrical cycles.

You can get UL489 breakers, installed in UL891 equipment, which have selectable Instantaneous settings including OFF. When the Inst function is turned off, there will still be an integral 'self-protection', or override, point to comply with the 3 cycle issue. The override has not traditionally been shown on breaker TCCs, although it seems more manufacturers are beginning to make note of it because it needs to be considered for selective coordination.

 

[mbrooke]

You can get UL489 breakers, installed in UL891 equipment, which have selectable Instantaneous settings including OFF. When the Inst function is turned off, there will still be an integral 'self-protection', or override, point to comply with the 3 cycle issue. The override has not traditionally been shown on breaker TCCs, although it seems more manufacturers are beginning to make note of it because it needs to be considered for selective coordination.

This is why I ask

 

[steve66]

Though it might not, depends on the run and source Z.

Well, I meant it should put you within the short time "time frame". I doubt many breakers allow the short time delay to be set at more than 0.5 seconds (30 cycles).

If there is full fault current for 30 cycles and it doesn't trip the breaker on short time, the breaker probably isn't set right.

For lower fault currents (faults farther downstream), that just pushes the breaker into the long time rating, but that can happen even if the instantaneous trip is set without delay.

So my point was simply that it doesn't matter if instantaneous trip is disabled or not for faults over 30 cycles. They are beyond the instantaneous time frame, and into other parts of the trip curve.