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 Post subject: Re: Low arcing current?
PostPosted: Wed Feb 06, 2019 3:46 pm 

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PaulEngr wrote:
Keep in mind here that IEEE 1584-2002 and -2018 assumes that arcs are self-sustaining out to 2 seconds (for the general case) and coordination calculations assume that both bolted and arcing faults are self-sustaining in perpetuity without regard for consumption of bus bars or other effects that would naturally extinguish the arc.

I usually use the 2-second rule when running calcs and did in this case.
PaulEngr wrote:
Modern installations lately have tended towards making instantaneous protection user-selectable, the so-called "maintenance switch" which is very effective at solving this very real problem at very low cost, much more so than the very costly solutions of 87 relaying, ZSI, or arc flash relays.

Thanks Paul. Maintenance mode switches can be a big help, but they can't be used on fusible switches. It will be interesting to see what solutions manufacturers of large switches find to comply with NEC 240.67 starting in 2020.

In this case bus bars were eroded an inch or so at the bottom of the faulted distribution panel. I would think this might provide an indication as to the total arc energy involved.

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 Post subject: Re: Low arcing current?
PostPosted: Thu Feb 14, 2019 12:36 pm 
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PaulEngr wrote:
Keep in mind here that IEEE 1584-2002 and -2018 assumes that arcs are self-sustaining out to 2 seconds (for the general case) and coordination calculations assume that both bolted and arcing faults are self-sustaining in perpetuity without regard for consumption of bus bars or other effects that would naturally extinguish the arc.

I usually use the 2-second rule when running calcs and did in this case.
PaulEngr wrote:
Modern installations lately have tended towards making instantaneous protection user-selectable, the so-called "maintenance switch" which is very effective at solving this very real problem at very low cost, much more so than the very costly solutions of 87 relaying, ZSI, or arc flash relays.


Thanks Paul. Maintenance mode switches can be a big help, but they can't be used on fusible switches. It will be interesting to see what solutions manufacturers of large switches find to comply with NEC 240.67 starting in 2020.

In this case bus bars were eroded an inch or so at the bottom of the faulted distribution panel. I would think this might provide an indication as to the total arc energy involved.


Large switches are basically circuit breakers particularly in the 69 kV class but this might not be what you mean. I just replaced a very old 3000 A Federal Pacific fused Pringles style switch with an insulated case breaker. The overall cost wasn't all the bad and it solved the problem. At some voltages S&C has a product called the "Fault Fiter" which is effectively a smart fuse...it has a fuse link but uses a protective relay to fire it. I believe Cooper has a similar product for 12.5 kV except that with their product you supply the protective relay. I know that this particular fuse would be easy to incorporate a "maintenance switch" but not sure if we can externally change the curves in the relay on the S&C product. Sticking with medium voltage where often short circuit current and/or current in general is a problem, you can also go to using a recloser which is very tolerant of very high fault currents and relatively inexpensive for what it is. But honestly are you really going to be doing 3000+ A installations these days? It seems like the arc flash requirements drive you to split the loads up into smaller loads and/or increase the bus voltage to the point where you can keep the currents reasonable. 3000-6000 A switches are likely to quickly become a thing of the past with safer by design to consider.


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 Post subject: Re: Low arcing current?
PostPosted: Mon Feb 18, 2019 7:36 am 

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Thanks Paul. I should have clarified that I meant "large" in the context of the discussion of low-voltage systems.

I was wondering if someone might develop a low-voltage fuse something along the lines of the Fault-fiter. I have spoken with two fuse manufacturers about this NEC requirement and not had any indication that they plan to offer anything new. So I think you are probably correct in that 1000 amp + fusible protection will not be able to be used in many applications going forward. It would be terrific if someone would come up with a retro-fit option.

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 Post subject: Re: Low arcing current?
PostPosted: Mon Feb 18, 2019 7:38 am 

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I asked the utility company to provide accurate fault current at the transformer secondary (the utility owns the transformer), and explained that I need accurate minimum and maximum values because I'm doing an arc flash study. The offered the following:

On 13.8 kV primary side of transformer:
3-phase fault current = 7748 amps
Single line to ground fault current = 7462 amps
Z1= .0732 + J1.025 ohms
Z0= .6514 + J1.050 ohms
Transformer is protected with: 125 amp, S & C , SMU-20, standard speed fuses

Transformer Data:
2000 kVA
277/480 V secondary
X/R ratio = 6.33(typical)
%Z=5.8
Grounded WYE connected primary
Grounded WYE connected secondary

They included a statement that this is "close to the minimum this location would see".

Realizing that 7748 amps at 13.8 for a transformer of this size is close to an "infinite primary", I asked if they could provide actual minimum and maximum values and they said this is all they have to offer.

So I re-ran the IE calcs with this primary and transformer data and got roughly the same results as I did with the secondary fault current value they gave me originally.

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 Post subject: Re: Low arcing current?
PostPosted: Mon Feb 18, 2019 8:44 am 
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stevenal wrote:
Thanks both of you. I wonder if a valid approach where we can't get meaningful fault values would be to run a couple of cases or a range of values and take the worst case.

I re-ran the calculations with the utility fault current reduced to 50% of the value I was given. This resulted in a calculated bolted fault current of 31.6 kA and "protective device arcing fault current" of 10.8 kA. The maximum breaker clearing time with settings that were in place at the time for this current is 16 seconds and the fuse clearing time is 45 sec.


So when will the software companies take your normal inputs along with a range of fault currents, and provide an IE versus fault current curve that shows at a glance what and where the highest IE is?


As to for instance the graphical approach to arc flash, it's more useful to look at it when we don't have complete information. It can allow us to model what can happen downstream of an overcurernt protective device without a lot of complete data. For instance we could predict worst case incident energy almost purely based on the device and maybe some assumptions about downstream inductive or capacitive loads. But due to the downstream load issue, I doubt you are going to see manufacturer "maximum incident energy" data on a data sheet for a breaker any time soon.

You have to remember, and this is going back to the IEEE 1584-2002 edition, remember the full bolted fault/85% "rule"? the explanation given in IEEE 1584-2002 is that the actual incident energies vs. available fault currents that they measured in the test data showed a bimodal and long tail distribution. In other words most of the time they had fault currents close to a specific value. Occasionally they would get fault currents at another, quite a bit lower value. Still other times it was random and could be just about anything. By adding the step of calculating incident energy based on the available bolted fault current and at 85% of that value, they were able to capture the bimodal distribution and this boosted the confidence interval significantly to the point where the final model showed roughly a 90-95% confidence interval as far as predicting incident energy based on numerical modelling (boot strap type of an approach).

There is an obvious explanation for this. DC testing shows that there are two stable currents which are a little less than the maximum power transfer current (which never happens) and not equal to each other. So sometimes you get a stable arc at point A and sometimes at B. This explains the bimodal distribution. Still other times for various reasons the arcing current might end up at some other point and this is almost purely random so we get the long tail part of the distribution but based on the numerical results from IEEE 1584-2002 this seems to be rare enough not to get too concerned over it.

IEEE 1584-2018 changed the way that arcing current is calculated and they have a lot more laboratory data to work with. At least looking through a copy of draft 6 (D6) I didn't see any kind of numerical studies and validation supporting the new model except statements that it works better than ever. So either they've managed to capture more of the variability in the new model and thus the previous variability in the 2002 model has been eliminated, or it still exists but without discussion it is hard to follow how well the new model fits the data or where the chinks in the armor are at.


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 Post subject: Re: Low arcing current?
PostPosted: Tue Feb 19, 2019 7:18 am 
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Maintenance mode switches can be a big help, but they can't be used on fusible switches.


Fusible switches, with shunt trip mechanisms, can have Ground Fault and under voltage protection, so there is no reason that other protective relay functions could not be added. Of course you have to consider the operating time of the shunt trip mechanism, but the overall clearing time will still likely be faster than waiting for a fuse to melt with only a 2-3x fault level.


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 Post subject: Re: Low arcing current?
PostPosted: Tue Feb 19, 2019 9:36 am 
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I asked the utility company to provide accurate fault current at the transformer secondary (the utility owns the transformer), and explained that I need accurate minimum and maximum values because I'm doing an arc flash study. The offered the following:

On 13.8 kV primary side of transformer:
3-phase fault current = 7748 amps
Single line to ground fault current = 7462 amps
Z1= .0732 + J1.025 ohms
Z0= .6514 + J1.050 ohms
Transformer is protected with: 125 amp, S & C , SMU-20, standard speed fuses

Transformer Data:
2000 kVA
277/480 V secondary
X/R ratio = 6.33(typical)
%Z=5.8
Grounded WYE connected primary
Grounded WYE connected secondary

They included a statement that this is "close to the minimum this location would see".

Realizing that 7748 amps at 13.8 for a transformer of this size is close to an "infinite primary", I asked if they could provide actual minimum and maximum values and they said this is all they have to offer.

So I re-ran the IE calcs with this primary and transformer data and got roughly the same results as I did with the secondary fault current value they gave me originally.


So you have data for one, presumably the normal, utility system configuration. The utility is constantly maintaining the system, though, and to keep the lights on will have many possible alternate configurations. I think the graphical approach would useful for considering such alternate configurations.


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 Post subject: Re: Low arcing current?
PostPosted: Tue Feb 19, 2019 9:52 am 

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JBD,
Low-voltage switches typically have limited capacity to break fault current. For switches equipped with ground fault relays, UL requires an interrupting rating of 10 to 12x the nominal rating of the switch, or an "integral means to prevent disconnecting at levels of fault current exceeding their contact-interrupting capability."

The integral means is usually fuses. When fuses are used to achieve this interrupting capacity, they must be coordinated with the ground fault relay operation such that the switch is not tripped when fault current is in excess of the contact ratings. This is one reason ground fault relays on switches usually have a definite minimum operating time.

The purpose of a maintenance mode switch is to interrupt fault current as quickly as possible to minimize IE exposure. That is outside of the capabilities of most low-voltage switches.

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 Post subject: Re: Low arcing current?
PostPosted: Tue Feb 19, 2019 10:15 am 

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stevenal wrote:
So you have data for one, presumably the normal, utility system configuration. The utility is constantly maintaining the system, though, and to keep the lights on will have many possible alternate configurations. I think the graphical approach would useful for considering such alternate configurations.


Right. I agree. Another approach is to run multiple cases such as a 50% scenario.

But getting back to my original question - even at 50% of this bolted fault current, the calculated arcing current is much higher than implied by the lack of operation of the overcurrent protection in this case. Why is that? I see two likely answers: (a) Either the actual fault current from the utility is grossly lower than they are saying, or (b) this case was a statistical aberration that did not fall within norms established by testing in development of IEEE 1584.

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 Post subject: Re: Low arcing current?
PostPosted: Tue Feb 19, 2019 11:14 am 
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JBD,
Low-voltage switches typically have limited capacity to break fault current. For switches equipped with ground fault relays, UL requires an interrupting rating of 10 to 12x the nominal rating of the switch, or an "integral means to prevent disconnecting at levels of fault current exceeding their contact-interrupting capability."

The integral means is usually fuses. When fuses are used to achieve this interrupting capacity, they must be coordinated with the ground fault relay operation such that the switch is not tripped when fault current is in excess of the contact ratings. This is one reason ground fault relays on switches usually have a definite minimum operating time.

The purpose of a maintenance mode switch is to interrupt fault current as quickly as possible to minimize IE exposure. That is outside of the capabilities of most low-voltage switches.


These kinds of switches are basically circuit breakers without an integral trip unit. And backing fuses are well backing fuses. There is no reason not to use a "smart fuse" other than no market. The cost of a smart fused disconnect with the relaying has to be close to a breaker.


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 Post subject: Re: Low arcing current?
PostPosted: Tue Feb 19, 2019 2:26 pm 

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PaulEngr wrote:
These kinds of switches are basically circuit breakers without an integral trip unit.
I have to respectfully take exception to this. Low-voltage switches are generally not designed to interrupt fault current.
PaulEngr wrote:
The cost of a smart fused disconnect with the relaying has to be close to a breaker.
I agree. It probably would be if it was available.

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 Post subject: Re: Low arcing current?
PostPosted: Tue Feb 19, 2019 4:02 pm 

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Low-voltage switches are generally not designed to interrupt fault current.

I'll amend this to say "Low-voltage switches are generally not designed to interrupt high fault current." Some are designed to interrupt ground faults at levels below their associated fuse curves.

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 Post subject: Re: Low arcing current?
PostPosted: Tue Feb 19, 2019 7:56 pm 
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Low-voltage switches are generally not designed to interrupt fault current.

I'll amend this to say "Low-voltage switches are generally not designed to interrupt high fault current." Some are designed to interrupt ground faults at levels below their associated fuse curves.


I've seen plenty of pole mounted automated switches that meet your description of a "switch". That is they usually have very low switching current limitations since they are load break but not fault breaking.

But I haven't seen something similar in the low voltage (480 V) world that would belong in say a switchgear lineup except for ATS's (automatic transfer switches) commonly used in generator applications, and PDU's (Power Distribution Units) commonly used for remote/automated server control. Typically whenever I see an industrial version that belongs in switchgear or MCC's that would be for instance a motor bus switch. This is a main breaker for an MCC which controls a bank of motors. The "breaker" often has no actual trip unit and operates purely by shunt close/trip. This allows it to be used for safety interlocks such as an E-Stop where the entire bank of motors can be shut down via one control signal instead of using individual relay contacts for each starter (the "MCR" approach).


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 Post subject: Re: Low arcing current?
PostPosted: Wed Feb 20, 2019 8:48 am 

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PaulEngr wrote:
The "breaker" often has no actual trip unit and operates purely by shunt close/trip.
Right. This is called a molded-case switch. As you say, these are basically a molded-case circuit breaker without a traditional trip unit. They usually do have an instantaneous trip element set at 10 to 12x the frame rating, designed only to protect the switch itself. I don't know if anyone offers an arc-flash maintenance switch in combination with an MCS, but it seems like it would theoretically be possible.

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 Post subject: Re: Low arcing current?
PostPosted: Wed Feb 20, 2019 11:44 am 
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PaulEngr wrote:
These kinds of switches are basically circuit breakers without an integral trip unit.
I have to respectfully take exception to this. Low-voltage switches are generally not designed to interrupt fault current.
PaulEngr wrote:
The cost of a smart fused disconnect with the relaying has to be close to a breaker.
I agree. It probably would be if it was available.


An arc flash energy reduction function could still operate within the 'non-fused' interrupting range of the switch, which is typically 10kA. That is why I specifically mentioned a protection level of 2-3X rated current.

In 2017 the NEC added section 240.67, in effect Jan 2020, which requires 600V fusible switches, 1200A and larger, to be supplied with some type of energy reduction. The language is similar to the 240.87 requirement for breakers.
In 2020 the NEC will likely add requirements for performance testing of these energy reduction methods. I do not see how a fuse will be able to be performance tested and then put into service.


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 Post subject: Re: Low arcing current?
PostPosted: Thu Feb 21, 2019 4:15 pm 
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JBD wrote:
PaulEngr wrote:
These kinds of switches are basically circuit breakers without an integral trip unit.
I have to respectfully take exception to this. Low-voltage switches are generally not designed to interrupt fault current.
PaulEngr wrote:
The cost of a smart fused disconnect with the relaying has to be close to a breaker.
I agree. It probably would be if it was available.


An arc flash energy reduction function could still operate within the 'non-fused' interrupting range of the switch, which is typically 10kA. That is why I specifically mentioned a protection level of 2-3X rated current.

In 2017 the NEC added section 240.67, in effect Jan 2020, which requires 600V fusible switches, 1200A and larger, to be supplied with some type of energy reduction. The language is similar to the 240.87 requirement for breakers.
In 2020 the NEC will likely add requirements for performance testing of these energy reduction methods. I do not see how a fuse will be able to be performance tested and then put into service.


Partial function testing as you would do on any other safety system. Obviously a fusible link itself can't be tested or has to be type tested which is what UL does for Listing purposes. But you could test the trip relay for instance if you lift the wire. You can't really even true performance test an arc flash tripping mechanism which is one of the allowable devices without type testing due to the expense. You just use a camera flash which isn't the same thing but we're down to functional testing vs. type testing. Fuses would work the same way. This is like the arguments of using secondary injection testing and verifying accuracty of the CT's over primary injection testing.


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 Post subject: Re: Low arcing current?
PostPosted: Tue Feb 26, 2019 1:27 pm 

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JBD wrote:
PaulEngr wrote:
These kinds of switches are basically circuit breakers without an integral trip unit.
I have to respectfully take exception to this. Low-voltage switches are generally not designed to interrupt fault current.
PaulEngr wrote:
The cost of a smart fused disconnect with the relaying has to be close to a breaker.
I agree. It probably would be if it was available.


An arc flash energy reduction function could still operate within the 'non-fused' interrupting range of the switch, which is typically 10kA. That is why I specifically mentioned a protection level of 2-3X rated current.

In 2017 the NEC added section 240.67, in effect Jan 2020, which requires 600V fusible switches, 1200A and larger, to be supplied with some type of energy reduction. The language is similar to the 240.87 requirement for breakers.
In 2020 the NEC will likely add requirements for performance testing of these energy reduction methods. I do not see how a fuse will be able to be performance tested and then put into service.


Thanks for the clarification. And I agree with your 2-3x assessment. I recall asking one switch manufacturer about interrupting rating and they would not provide a direct answer. I don't have the UL standard and don't know what is actually required. I do know that the switch has to be tested in combination with fuses.

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 Post subject: Re: Low arcing current?
PostPosted: Tue Feb 26, 2019 1:55 pm 

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PaulEngr wrote:
Partial function testing as you would do on any other safety system. Obviously a fusible link itself can't be tested or has to be type tested which is what UL does for Listing purposes. But you could test the trip relay for instance if you lift the wire. You can't really even true performance test an arc flash tripping mechanism which is one of the allowable devices without type testing due to the expense. You just use a camera flash which isn't the same thing but we're down to functional testing vs. type testing. Fuses would work the same way. This is like the arguments of using secondary injection testing and verifying accuracty of the CT's over primary injection testing.

There is a "public comment" proposal to add a requirement in 240.67 for field testing of arc energy reduction systems utilizing primary injection testing. There is a similar proposal for 240.87.

Another proposal modifies the wording to only require arc energy reduction systems where work on energized systems is permitted.

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 Post subject: Re: Low arcing current?
PostPosted: Sun Mar 03, 2019 12:05 am 
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How do you not work on energized systems? Testing for absence of voltage comes to mind. At one time 70E had three different LOTO procedures. They had a simple LOTO, a complex LOTO similar to OSHA, and a cord-and-plug "LOTO" which was really just pulling the plug out. The latter has been removed. So now we test everything? And that is energized work. So the exceptions aren't necessary.


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 Post subject: Re: Low arcing current?
PostPosted: Wed Feb 26, 2020 10:47 am 

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Hi...I didn't realize the fault current through the upstream device that is tripping would be different, but now that I think about it, it makes sense. I had read the equations of IEEE 1582 on how to calculate the arc fault current, but I didn't realize it also had equations for determining the protective device arcing fault current, which is based on the calculated arcing fault current which is in turn based on the bolted fault current.


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