Using general purpose relays downstream to current limiting fuses

LMAO

Senior Member
Location
Texas
Hello, it may be a dumb question but can I use general purpose relays downstream to a current limiting fuse? The reason I need the fuse is that I have 50kA available fault on 480V line which is used to control a 480V coil. Short circuit withstand rating for the general purpose relay contactor used to energize the coil is 5kA and fuse RMS let through at 50kA seems to be less than 2000A.

Fuse curve (30A)
Relay

Am I missing something? I expected this to be more difficult.
Thanks
 
We have had some long debates on this. While there is no question that you can't use a current limiting fuse to lower the available fault current of a downstream ocpd, I don't see any reason you can't do this to lower the SCCR seen by a downstream piece of equipment that is not an OCPD.
 

LMAO

Senior Member
Location
Texas
We have had some long debates on this. While there is no question that you can't use a current limiting fuse to lower the available fault current of a downstream ocpd, I don't see any reason you can't do this to lower the SCCR seen by a downstream piece of equipment that is not an OCPD.
yeah I remember bumping into this argument now and then, that despite OCPD manufacturers claims of current limiting you can't really use it with another OCPD unless the two are tested together.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
The data published by the fuse mfrs is intended for people who are going to produce a TESTED AND LISTED COMBINATION, because they want to know before they spend the $50k or so for the testing that it is going to pass. You cannot apply that data in the field, unless you are a registered PE and are willing to stake your reputation on it. It might be technically correct, but it does NOT change the SCCR listing of the device or panel, and it does NOT lower the Available Fault Current.
 

mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
If doing the evaluation per NEC 240.86 Series Ratings. Here's what Jraef is referring to: from NFPA Link

Where a circuit breaker is used on a circuit having an available fault current higher than the marked interrupting rating by being connected on the load side of an approved overcurrent protective device having a higher rating, the circuit breaker shall meet the requirements specified in 240.86(A) or (B), and (C).
(A) Selected Under Engineering Supervision in Existing Installations.
The series rated combination devices shall be selected by a licensed professional engineer engaged primarily in the design or maintenance of electrical installations. The selection shall be documented and stamped by the professional engineer. This documentation shall be available to those authorized to design, install, inspect, maintain, and operate the system. This series combination rating, including identification of the upstream device, shall be field marked on the end use equipment.
For calculated applications, the engineer shall ensure that the downstream circuit breaker(s) that are part of the series combination remain passive during the interruption period of the line side fully rated, current-limiting device.

But don't use let-through curves. Instead us let-through tables like shown below.
1631636570311.png
This is to be able to accommodate power source upgrades for existing installations.
 

LMAO

Senior Member
Location
Texas
The data published by the fuse mfrs is intended for people who are going to produce a TESTED AND LISTED COMBINATION, because they want to know before they spend the $50k or so for the testing that it is going to pass. You cannot apply that data in the field, unless you are a registered PE and are willing to stake your reputation on it. It might be technically correct, but it does NOT change the SCCR listing of the device or panel, and it does NOT lower the Available Fault Current.
If doing the evaluation per NEC 240.86 Series Ratings. Here's what Jraef is referring to: from NFPA Link

Where a circuit breaker is used on a circuit having an available fault current higher than the marked interrupting rating by being connected on the load side of an approved overcurrent protective device having a higher rating, the circuit breaker shall meet the requirements specified in 240.86(A) or (B), and (C).
(A) Selected Under Engineering Supervision in Existing Installations.
The series rated combination devices shall be selected by a licensed professional engineer engaged primarily in the design or maintenance of electrical installations. The selection shall be documented and stamped by the professional engineer. This documentation shall be available to those authorized to design, install, inspect, maintain, and operate the system. This series combination rating, including identification of the upstream device, shall be field marked on the end use equipment.
For calculated applications, the engineer shall ensure that the downstream circuit breaker(s) that are part of the series combination remain passive during the interruption period of the line side fully rated, current-limiting device.

But don't use let-through curves. Instead us let-through tables like shown below.
View attachment 2557805
This is to be able to accommodate power source upgrades for existing installations.
Thank you, this is very informative.

So to be clear, this only applies to two OCPDs used in series, correct? In other words, I should not have any problem using a simple contactor rated for 5kA downstream to a current limiting fuse with RMS let through of 2kA where available fault current is 50kA, right?
 

mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
Thank you, this is very informative.

So to be clear, this only applies to two OCPDs used in series, correct? In other words, I should not have any problem using a simple contactor rated for 5kA downstream to a current limiting fuse with RMS let through of 2kA where available fault current is 50kA, right?
I think the let-through charts apply to everything, including buses and the contactor you're referencing.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Thank you, this is very informative.

So to be clear, this only applies to two OCPDs used in series, correct? In other words, I should not have any problem using a simple contactor rated for 5kA downstream to a current limiting fuse with RMS let through of 2kA where available fault current is 50kA, right?
No. I don't know how you got there.

Unless you are a licensed PE, you cannot use fuse let through curves to change the SCCR of the relay / contactor. Your contactor is only good for 5kA, you cannot change that. Your circuit has 50kA available. It cannot be installed that way.

Here are your ONLY choices:
  1. If the CONTACTOR manufacturer has listed their contactor in a series combination with SPECIFIC fuses at 50kA SCCR or greater, you can use those fuses ahead of it. Be prepared to document that to an AHJ.
  2. Choose a contactor that DOES have a listing at 50kA or greater with a specific fuse (or other OCPD) ahead of it. Be prepared to document that to an AHJ.
  3. Add a transformer or reactor of sufficient impedance to bring the Available Fault Current down to 5kA or less.
  4. Seek, find and hire a PE willing to stake his reputation and licensing on certifying that a fuse combination is going to be sufficient to protect that cheap little contactor (hint, you won't find that person).
  5. Have a UL508A shop build you a control panel using components that are series listed by the manufacturer for 50kA or more and put a label on it saying that the SCCR is that level (same as #2 really, but more official and avoids having to document it).
 

EC Dan

Member
Location
Florida
Occupation
E&C Manager
No. I don't know how you got there.

Unless you are a licensed PE, you cannot use fuse let through curves to change the SCCR of the relay / contactor. Your contactor is only good for 5kA, you cannot change that. Your circuit has 50kA available. It cannot be installed that way.

Here are your ONLY choices:
  1. If the CONTACTOR manufacturer has listed their contactor in a series combination with SPECIFIC fuses at 50kA SCCR or greater, you can use those fuses ahead of it. Be prepared to document that to an AHJ.
  2. Choose a contactor that DOES have a listing at 50kA or greater with a specific fuse (or other OCPD) ahead of it. Be prepared to document that to an AHJ.
  3. Add a transformer or reactor of sufficient impedance to bring the Available Fault Current down to 5kA or less.
  4. Seek, find and hire a PE willing to stake his reputation and licensing on certifying that a fuse combination is going to be sufficient to protect that cheap little contactor (hint, you won't find that person).
  5. Have a UL508A shop build you a control panel using components that are series listed by the manufacturer for 50kA or more and put a label on it saying that the SCCR is that level (same as #2 really, but more official and avoids having to document it).

Excellent rundown of this. One clarifying question, can the fuses and peak let-throughs in Table SB4.2 of UL508A (being an approved method for determining SCCR in NEC) be applied without a PE? This does not rely on interpretation of let-through curves.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Excellent rundown of this. One clarifying question, can the fuses and peak let-throughs in Table SB4.2 of UL508A (being an approved method for determining SCCR in NEC) be applied without a PE? This does not rely on interpretation of let-through curves.
That table can only be used by a UL508A panel shop.
 
No. I don't know how you got there.

Unless you are a licensed PE, you cannot use fuse let through curves to change the SCCR of the relay / contactor. Your contactor is only good for 5kA, you cannot change that. Your circuit has 50kA available. It cannot be installed that way.

Here are your ONLY choices:
  1. If the CONTACTOR manufacturer has listed their contactor in a series combination with SPECIFIC fuses at 50kA SCCR or greater, you can use those fuses ahead of it. Be prepared to document that to an AHJ.
  2. Choose a contactor that DOES have a listing at 50kA or greater with a specific fuse (or other OCPD) ahead of it. Be prepared to document that to an AHJ.
  3. Add a transformer or reactor of sufficient impedance to bring the Available Fault Current down to 5kA or less.
  4. Seek, find and hire a PE willing to stake his reputation and licensing on certifying that a fuse combination is going to be sufficient to protect that cheap little contactor (hint, you won't find that person).
  5. Have a UL508A shop build you a control panel using components that are series listed by the manufacturer for 50kA or more and put a label on it saying that the SCCR is that level (same as #2 really, but more official and avoids having to document it).
I'm not sure I agree. Note 240.86 ONLY APPLIES TO TWO OCPD'S IN SERIES. When the second device is passive, I don't see why the current limiting fuse won't lower the available fault current to the let through value shown by the fuse manufacturers charts.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
By putting a contactor in a box to control something, the OP has created an "Industrial Control Panel as defined in Article 409.

409.1 Scope. This article covers industrial control panels
intended for general use and operating at 1000 volts or less.
Informational Note: ANSIIUL 508, Standard for Industrial
Control Panels, is a safety standard for industrial control
panels.
409.3 Other Articles. In addition to the requirements of
Article 409, industrial control panels that contain
branch circuits for specific loads or components, or are for
control of specific types of equipment addressed in other
articles of this Code, shall be constructed and installed in
accordance with the applicable requirements from the specific
articles in Table 409.3
.
Then in 409 is where we find the provisions calling for having an SCCR rating.
409.110 Marking. An industrial control panel shall be
marked with the following information that is plainly visible
after installation: ...
(4) Short-circuit current rating of the industrial control
panel based on one of the following:
a. Short-circuit current rating of a listed and labeled
assembly

b. Short-circuit current rating established utilizing an
approved method

Informational Note: ANSI/UL 508, Standard for Industrial
Control Panels, Supplement SB, is an example of an
approved method.
Or if you want to just call it industrial machinery under Article 670, these sections are repeated there.
Nowhere is looking up a fuse let-through chart listed as an approved method.

Or, if you want to argue that it is not an "industrial control panel" or "industrial machinery", then it is still a "controller".
430.8 Marking on Controllers. A controller shall be
marked with the manufacturer's name or identification, the
voltage, the current or horsepower rating, the short-circuit
current rating
, and· other necessary data to properly indicate
the applications for which it is suitable.
He has a "controller" that has an SCCR of 5kA. He cannot change what it is marked as.
He has an Available Fault Current of 50kA. The fuse does not change the Available Fault Current.
The fuse let-through issue might WORK in a LISTED ASSEMBLY, but that is not what he has here. He is putting parts together in the field, that is not considered an "acceptable method".
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
As I understand it, the Assembly is considered listed and labeled as long as UL has approved the assembly regardless of who puts it together. Otherwise you could not buy a contactor and mate it to an overload.

So if UL says a particular combination of fuse and contactor is good for a certain short circuit current rating when used together, my understanding is that is acceptable.

What you cannot do is use some of the provisions of UL 508A that go beyond this such as using the let-through current of a current limiting fuse.
 
By putting a contactor in a box to control something, the OP has created an "Industrial Control Panel as defined in Article 409.


Then in 409 is where we find the provisions calling for having an SCCR rating.

Or if you want to just call it industrial machinery under Article 670, these sections are repeated there.
Nowhere is looking up a fuse let-through chart listed as an approved method.

Or, if you want to argue that it is not an "industrial control panel" or "industrial machinery", then it is still a "controller".

He has a "controller" that has an SCCR of 5kA. He cannot change what it is marked as.
He has an Available Fault Current of 50kA. The fuse does not change the Available Fault Current.
The fuse let-through issue might WORK in a LISTED ASSEMBLY, but that is not what he has here. He is putting parts together in the field, that is not considered an "acceptable method".
Jraef, I still find this to be kinda of vague. I really dont have a dog in this fight, I just feel they need to be clearer on this issue. Perhaps they need to provide a definition for "Available fault current". If you look at the definition of "current limiting overcurrent protective device" it seems to say you can use a CL fuse just like, say, a transformer to reduce the available fault current. We seem to be fine with using a transformer to reduce the AFC but no a CL fuse, and the NEC is silent on this, perhaps they should be more specific.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
The issue here is not whether Current Limiting Fuses will work to allow a panel to be marked with a higher SCCR than the individual components can carry by themselves, they DO provide that function. The issue is that the assembly (control panel, stand alone starter, etc.) must be LABELED through an APPROVED METHOD at an SCCR level commensurate with the Available Fault Current. Contractors in the field using a fuse let-through chart is NOT AN APPROVED METHOD. The manufacturer's listing is, as is the listing and labeling process of an NRTL listed panel shop. Do the fuses "change" the available fault current of the system? Nothing I have ever seen indicates that they do. AFC (also called PSCC) is a function of impedance, voltage and time.
 
Contractors in the field using a fuse let-through chart is NOT AN APPROVED METHOD.

. Do the fuses "change" the available fault current of the system? Nothing I have ever seen indicates that they do. AFC (also called PSCC) is a function of impedance, voltage and time.

If that is the case then the NEC needs to be clear on it, and be specific about what an "approved method" is. If you look at the definition of "Current limiting overcurrent protection device", they seem to say it does exactly that, reduces the available fault current, and the manufacturers provide charts for this.
 
Top