Disconnects Vs Fused Disconnects

[wbdvt]

But my point is that most 30A equipment discos will have a 10k breaker upstream so there is no problem using an unfused disco. And by 'most' I mean 99.9%, at least in resi.

Just because there is a 10kA rated breaker upstream has no bearing on what the fault current is. That 10kA breaker may be series rated with the main breaker in the panel to be 22kA. So if you are saying that since a 10kA breaker is upstream from a non-fused disco, you are ok? Suppose the fault current is 12kA and that is why there is a series rated panel for the 10kA breakers to be rated higher.

The point being unless some analysis is done of the system, you really have no idea if the non-fused disco is overdutied. An overdutied condition is a violation of NEC and OSHA.

When I do a study that involves arc flash, our labels have the available short circuit current for the equipment being labeled, printed right on the label. This way the electrician that may be replacing a breaker or installing a new circuit, can verify that the breaker purchased is adequate for the use.

 

[jaggedben]

Just because there is a 10kA rated breaker upstream has no bearing on what the fault current is. That 10kA breaker may be series rated with the main breaker in the panel to be 22kA. So if you are saying that since a 10kA breaker is upstream from a non-fused disco, you are ok? Suppose the fault current is 12kA and that is why there is a series rated panel for the 10kA breakers to be rated higher.

The point being unless some analysis is done of the system, you really have no idea if the non-fused disco is overdutied. An overdutied condition is a violation of NEC and OSHA.

When I do a study that involves arc flash, our labels have the available short circuit current for the equipment being labeled, printed right on the label. This way the electrician that may be replacing a breaker or installing a new circuit, can verify that the breaker purchased is adequate for the use.

Fair enough, but a couple questions and points.

First, I'm not sure it's a violation of the NEC. What's the section? And if you cite 110.9, is a typical residential disconnect (say, for an air-conditioner) 'intended to interrupt current at fault levels'?
Second, does OSHA apply to residential installations? (i.e. to what is actually installed, as opposed to safety of installation workers while installing)

Finally, there are literally millions upon millions of residential air-conditioning circuits with unfused 10 kAIC disco behind non-series rated OCPDs, and it doesn't seem to be that much of a problem.

 

[wbdvt]

A residential disconnect or any non fused disconnect is not intended to interrupt current at fault levels so you need to understand the difference between a device that is designed to interrupt fault current (e.g. breaker or fuse) and a device that needs to withstand the level of fault current during the time it takes the OCPD to operate and clear a fault. This is why disconnects, panels, etc. have an SCCR which cannot be exceeded.

If you have the NEC handbook, there is commentary for 110.9 and 110.10 concerning short circuits and equipment ratings. From 110.9:

Fuses or circuit breakers that do not have adequate interrupting ratings could rupture while attempting to clear a short circuit. The interrupting rating of an overcurrent protective device is determined under standard test conditions. The rating should meet or exceed the actual installation needs. Interrupting ratings should not be confused with short-circuit ratings.

From 110.10: Wire, bus structures, switching, protection and disconnect devices, and distribution equipment all have limited short-circuit ratings and would be damaged or destroyed if those short-circuit ratings were exceeded. Merely providing overcurrent protective devices with sufficient interrupting ratings would not ensure adequate short-circuit protection for the system components."

OSHA does not apply to residential installations or work but merely pointing out that OSHA does contain items found in the NEC.

And just because there are installations out there, does not mean it is right or correct. Also imagine installing an under-rated disconnect, a fault happens, the disconnect explodes, catches the residence on fire and the resulting investigation reveals the cause to be the under-rated disconnect. The lawyer will have field day with you in a lawsuit.

 

[electrofelon]

Fair enough, but a couple questions and points.

First, I'm not sure it's a violation of the NEC. What's the section? And if you cite 110.9, is a typical residential disconnect (say, for an air-conditioner) 'intended to interrupt current at fault levels'?
Second, does OSHA apply to residential installations? (i.e. to what is actually installed, as opposed to safety of installation workers while installing)

Finally, there are literally millions upon millions of residential air-conditioning circuits with unfused 10 kAIC disco behind non-series rated OCPDs, and it doesn't seem to be that much of a problem.

A big part of it is, Probably 99.7% of dwellings have an available fault current under 10k we often assume its not an issue. Even in the case of the POCO using a large, say, 100kva transformer because its serving a bunch of houses, even then you likely will be below 10k even at the service equipment, and of course lots lower by the time you get to an AC disconnect.. A disclaimer however, many people forget or dont know that for single phase, the L-N fault current is typically higher than the L-L by a factor of 1.5.

 

[Klockopotomis]

Just a general question for you EC's out there.
If you have a 30A/2 piece of equipment, do you see more 30A/2 Disconnects or 30A/2 Fused disconnects in the field? I know code required just a disconnect, but its our company standard to provide most all with fused disconnects... Just seeing if this is an industry standard to protect equipment or an over engineered standard.

Thanks,

Every company I’ve worked for stocks fuseless disco’s for almost all equipment. In my company now, we only use fuseless.

 

[wwhitney]

for single phase, the L-N fault current is typically higher than the L-L by a factor of 1.5.

Can you explain why?

Thanks, Wayne

 

[electrofelon]

Can you explain why?

Thanks, Wayne

I believe because with a center tapped winding, the impedance line to neutral will be half resulting in theoretically twice the fault current if you use the line the line impedance?

See this paper for me info

https://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/fuses/solution-center/bus-ele-tech-lib-electrical-formulas.pdf

 

[jim dungar]

I believe because with a center tapped winding, the impedance line to neutral will be half resulting in theoretically twice the fault current if you use the line the line impedance?

This condition occurs at the secondary terminals of a transformer.
After a relatively short distance the L-N fault current drops below the L-L value.

See the complete text of the footnote in the referenced material.

 

[jaggedben]

A residential disconnect or any non fused disconnect is not intended to interrupt current at fault levels so you need to understand the difference between a device that is designed to interrupt fault current (e.g. breaker or fuse) and a device that needs to withstand the level of fault current during the time it takes the OCPD to operate and clear a fault. This is why disconnects, panels, etc. have an SCCR which cannot be exceeded.

If you have the NEC handbook, there is commentary for 110.9 and 110.10 concerning short circuits and equipment ratings. From 110.9:

Fuses or circuit breakers that do not have adequate interrupting ratings could rupture while attempting to clear a short circuit. The interrupting rating of an overcurrent protective device is determined under standard test conditions. The rating should meet or exceed the actual installation needs. Interrupting ratings should not be confused with short-circuit ratings.

From 110.10: Wire, bus structures, switching, protection and disconnect devices, and distribution equipment all have limited short-circuit ratings and would be damaged or destroyed if those short-circuit ratings were exceeded. Merely providing overcurrent protective devices with sufficient interrupting ratings would not ensure adequate short-circuit protection for the system components."

OSHA does not apply to residential installations or work but merely pointing out that OSHA does contain items found in the NEC.

And just because there are installations out there, does not mean it is right or correct. Also imagine installing an under-rated disconnect, a fault happens, the disconnect explodes, catches the residence on fire and the resulting investigation reveals the cause to be the under-rated disconnect. The lawyer will have field day with you in a lawsuit.

Do non-fused discos have lower SCCR than fused discos? Also I thought we were talking about kAIC not SCCR.

Do common toggle switches have sccr ratings? Why aren't they all fused?

A lawyer can only have a 'field day' if they can readily establish that I knew or should have known that the available fault current exceeded the equipment rating. If I can claim that it is reasonable to assume that the equipment is designed for the conditions it is commonly marketed and used for, that’s a mitigating factor, and I can probably point the finger at the manufacturer. (Also, for what it's worth, in my experience the technical merits aren't really all that relevant when the situation involves lawyers.)

 

[EC Dan]

Do non-fused discos have lower SCCR than fused discos? Also I thought we were talking about kAIC not SCCR.

Do common toggle switches have sccr ratings? Why aren't they all fused?

SCCR is for non-interrupting equipment, AIC is for interrupting devices. The fuse in the fused disconnect turns it into an interrupting device. Both tell you how much fault current can be safely tolerated by the system. The SCCR is based on NRTL testing, however if the manufacturer did not pay to have their equipment tested, the equipment may default to the values listed in the table below based on its classification. A toggle switch is likely not going to have a tested rating, therefore it would default to 5 kA SCCR based on the table below (Switch unit). A toggle switch is likely to be used in a control circuit which is not subject to SCCR limitations.

 

[wbdvt]

Do non-fused discos have lower SCCR than fused discos? Also I thought we were talking about kAIC not SCCR.

Do common toggle switches have sccr ratings? Why aren't they all fused?

A lawyer can only have a 'field day' if they can readily establish that I knew or should have known that the available fault current exceeded the equipment rating. If I can claim that it is reasonable to assume that the equipment is designed for the conditions it is commonly marketed and used for, that’s a mitigating factor, and I can probably point the finger at the manufacturer. (Also, for what it's worth, in my experience the technical merits aren't really all that relevant when the situation involves lawyers.)

Well, from my experience in participating in legal electrical matters, a lawyer will point to you as being a qualified individual, based on your experience and license (I assume you are licensed), who should know the limitations of the equipment and requirements for installation in the system. It is also on the installer to follow manufacturer's instructions on installation and many instruction sheets have a disclaimer about the equipment being installed in a manner it is not designed for. Next time you have a switch, read the instructions packed with it. So yes the equipment is designed for a specific application/system parameters, so it is up to the designer or installer to ensure that it is installed according to manufacturer instructions and UL listing. So no finger will pointed at the manufacturer.

And I am not going to beat a dead horse anymore on this matter. I just hope something was learned from this entire thread that will improve understanding of AIC and SCCR ratings and improve the safety of installations.