Be careful when matching fuses with circuit breakers. Circuit breakers are thermal magnetic and fuses are thermal. They both have different current clearing characteristics. To do it properly go to the manufacturer and find out what fuse can be matched with the circuit breaker you are using or overlap the current characteristic curves of the two devices.
110.9, 110.10. I've had mixed answers on this topic from both electrical experts and equipment manufacturers. I've run into this problem specifically with HVAC equipment, where the proposed or installed equipment had an interrupting rating that was lower than what was available at the line terminals of the equipment. My first step was to go back to the manufacturer to see if they could make any field modifications. Some manufacturers sell a kit that does not violate the listing and it can be used to increase SCCR. But like I say - I get mixed answers with regard to this topic and from all the big players (Siemens, Bussman, Schneider, and all the rest). I try to catch these issues on plan review. This is a perfect example of why we need plan review.
Not sure what equipment you are talking about but most branch circuits [ not feeders ] could not generate a fault current high enough to cause significant damage. The idea of bracing is to contain a fault and prevent the material from exploding. NEC 110.9 and 110.10. Control panels for example do not have an interrupting ratings but there are a wide variety of fuses that can be used if dealing with sensitive electronics.
Here is a situation:
500KVA utility transformer 1% impedance at 208/120V 3PH 4W 100 foot feeder to a 1400A MDP with four sets of 500KCM copper the fault current is about 66,000amps considering infinite primary.
From the MDP to a 225Amp branch panel with 50feet of 4/0copper the fault current is 25,000amps
From branch panel to a 30amp device with 100feet of 10AWG copper the fault current is 1100amps.
From branch panel to a 60amp device with 100feet of 4AWG copper the fault current is 3800amps.
Keep in mind this calculation is based on a bolted fault connection which is very rare in real life by accident.
If you are concerned coil the wire up with several loops and make a reactor out of it. Reactors will limit fault current but can create starting issues for motors.
Ron,
Are you saying that for Sensitive Electronics tied to the same Ground as a 1/2 MegaVA Xfmr, a Static Bracelet should be bolted to Ground with 10AWG (<100Ft and coiled for Reactance) having an In-Line 1A Fuse (2kA Interrupting Rating as the question asks) so long as the 30A Panel Breaker has a rating of 10+kA interrupting rating?
From Robert
Ron,
Are you saying that for Sensitive Electronics tied to the same Ground as a 1/2 MegaVA Xfmr, a Static Bracelet should be bolted to Ground with 10AWG (<100Ft and coiled for Reactance) having an In-Line 1A Fuse (2kA Interrupting Rating as the question asks) so long as the 30A Panel Breaker has a rating of 10+kA interrupting rating.
Response: Robert where do you find this stuff? 1/2MegaVA XFMR? Why not say it as it is usually stated as a 500KVA transformer. I can't give you a full theoretical study in lightning, lightning control and short circuits. I was merely giving you and example of fault current magnitudes using specific conductors of a specific length in a net work. I would recommend that you get some knowledge of short circuit currents and how they are produced in both AC and DC circuits. There are many types and sizes of transformers and each one will effect short circuit current. Remember short circuit calculations are based on a bolted fault condition and usually apply to AC circuits. [ taking two apposing phases and literally bolt them together, which rarely ever happens in real life by itself. ]
FYI: KW is a load with unity power factor = to 1 such as an electric heater and KVA is load associated with motors perhaps with a power factor of .85. The two should not be confused. Sizing electrical systems should be based on KVA load not KW load. OK lets move on to your comment.
An anti static ESD wrist strap usually clips onto a grounding buss or stud with alligator clips near the work station within 5feet. There could be many work stations in one room with several grounding points to clip onto. In this case a low impedance ground is not critical. This would mean some type of ground grid would need to be provided in the room to connect all the grounding stations in the room. I have used a lattice of bare braided and flat copper wire under a raised floor bonded to nearest building steel not to earth ground but I have also used building steel where available. The conductor between the wrist strap and the alligator clip is about 6 feet in length of small gauge. You apparently think that if a person was attached to an anti static ESD wrist strap and the wrist strap was attached to building steel via a ground grid that person could be zapped by lightning and are trying to get me to agree. Truth of the matter is no one is safe in a hail of lightning no matter where they are. However, I would look at the stats before casting judgment. There are precautions and poor choices a person can make when caught in a hail of lightning. Its better to get some knowledge on the subject before that happens. When dealing with current up to 300,000amps at a potential of up to one billion volts there are no guarantees. All we can do is try to minimize the damage caused by lightning energy. One bright star in the whole thing is that not many die from getting hit from lightning and there is a reason. All people hit by lightning are never struck by the main trunk but rather a feeder of considerably less magnitude and is true with buildings most of the time. As I said before electricity [ including lightning ] does not have a brain to pick its victims and always follows the laws of physics. If someone grounded himself to an air terminal on the roof of a building in a lightning storm he may be in jeopardy. I wouldn't recommend that by the way. Buildings are constructed of steel or steel reinforced concrete and as a result attract lightning. The goal of building lightning protection is to reduce the impedance to ground so that lightning takes a predicted course to ground to keep building destruction at a minimum. Making up would if scenarios is not realistic. That is like thinking because the woods are dark there must be monsters hiding there.
Ron,
What does a 500kVA XFMR have to do with a 1A Fuse?
What does Watts have to do with this conversation?
Why was "Lightning" brought into the conversation? Speaking of Lightning, it's "pulsed"' DC, bolted AC fault theory calculations do not apply.
I don't know of any flat Copper Wire. Perhaps you meant to say flat braided tinned Copper Straps. Even Faraday Screens are round Copper. Flat Copper is referred to Foil, Sheets or Plates.
Ron,
What does a 500kVA XFMR have to do with a 1A Fuse?
Response: I believe the question started like this: "You have a device (SCCR 5000A) on a branch circuit with standard circuit protection (say 20A, 1pole breaker, 10+ kA Interrupting rating) Can you use an inline fuse to protect the device with an interrupting rating of ...say something like 1000 or 2000 A? Where does the NEC cover something like this?"
Now what is being asked about is relating to fault current in an AC system and how to protect a device that has an interrupting rating less then the avialble fault current. I provided an illustration to show what happens to fault current in an AC electrical system from a 500KVA transformer all the way down to a small branch circuit. It shows that fault current is not an issue with small branch circuits unless the small branch circuit is connected to sensitive electronics in which case other methods would be used to mitigate high relative current. Fuses are inherently slow acting and would not clear a fault in time to save electronic componants. There are fast acting fuses for electronics that do a fair job but not a save all. The whole idea if withstandability is to keep the device from blowing up in someone faces, or damaging property. Try doing the research on what device has an AIC rating.
What does Watts have to do with this conversation?
Response: Watt is a unit of power at unity power factor by which amps are derived. Watts / volts = amps.
Why was "Lightning" brought into the conversation?
Response: You are someone else brought it into the conversation I was only making corrections in your understanding of lightning and grounding straps
Speaking of Lightning, it's "pulsed"' DC,
Response: Lightning is not pulsed DC it can be considered plasma but Pulsed DC and AC are very different things and do not apply to lightning.
bolted AC fault theory calculations do not apply.
Response: When ever you do a short circuit study on an AC system it is always with the idea of a bolted connection phase to phase or phase to ground. To do this would have to be an intentional act and I have had electricians do this without checking and then energy the system.
I don't know of any flat Copper Wire.
Response: AC current flows at the surface of a conductor not through the center. RF flows at the surface rather than through the wire, and flat braid has more surface area for any given current carrying capacity. It has more capacitive reactance which increases the self resonant point. Flat braid is also much more flexible and less likely to fail due to repeated flexing. A ground strap may work perfectly as a DC ground, but at some frequency that same ground strap will make a perfect antenna! We all know that an inductor can provide a good DC ground, but look like an open circuit to RF. A capacitor can provide a good RF path to ground, but not a DC path. Ground strap, like any piece of wire, has both inductive and capacitive reactance. These reactances change as the frequency changes. Very important to size flat grounding conductors properly.
Perhaps you meant to say flat braided tinned Copper Straps.
Response: No you can get flat braided copper, flat braided tinned copper, flat copper and flat braided aluminum. They all serve a specific purpose. You do realize that all lightning protection conductors [ copper and aluminum ] are braided for a reason. Under high voltage/current stress the strands of a conductor will try and separate and if it were not braid the wire would separate far enough and burn up making the system useless.
Even Faraday Screens are round Copper. Flat Copper is referred to Foil, Sheets or Plates.
Response: Not sure what book you are reading out of. Yes electronic terminology is very different than AC electrical terminology. Even the utility uses hollow conductor for their high voltage transmission lines
Hello. Thanks for your responses. I am not sure if my question was completely clear.
CB -------------- Device in branch circuit (5000 SCCR) -- Inline fuse (500A int.) ------------------------------------------------------
l l l l ..... load1 load2 load3 load4 ....
The current flowing in the circuit actually passes through the device and the inline fuse before power is delivered to the loads.
First thing you need to do is find out what the overcurrent device is capable of withstanding in a short circuit condition.
The device should state its maximum withstand rating. If the device is only capable of withstanding 10,000amps sym then the current needs to be limited to a value less then 10,000amps. There is such a thing as series ratings. The manufacturer would have that information. The manufacturer has a schedule that shows what devices can be connected in series to limit fault current to a specific value. If there are no schedules you will need to go to the device trip curves and find someone to teach you how to read them.
I posted an answer but it's locked up (being moderated)
Hi Nick,
As I have had recent experience with your same issue (being moderated) - I believe it has to do with pasting a Link.
I put the Link that I was trying to add as text without http or www and my responses were posted.
I've seen other Links posted?!? https://www.ecmweb.com/electrical-testing/voltage-loss-versus-voltage-drop
There's a Link Icon in the Text Editors above that I used for this one - it worked!
just tried again 2 different ways - post is locked out, "being moderated" I'm not sure why all the problem posting links...
Ok, the original 'reply' flagged as being moderated needs to be deleted, even though you can still make changes to it.
Try deleting the Links, highlight your reply, right click and Copy. Delete the entire Reply thru 'Actions'. Select 'Reply to original question', with the cursor inside, ctrl-V (to paste), add Links, then 'Add Comment' should do it.
I found out another hick-up, the Reply will be flagged yet again if the Link is an "https" (secured) so paste as text without the prefix.
My earlier answer is simple, and absolutely correct in nearly any instance. Manufacturers of the branch circuit panelboards, circuit breakers, and load devices have no say, no impact.
As long as your supplementary overcurrent protection device is placed on the load side of the branch circuit overcurrent protection, your only concerns are wiring that supplemental device in compliance with the code sections covering wiring, enclosures, accessibility, and the like.
Too simple? Apparently so, for this forum. Stand out from the crowd: don't make a mountain out of a molehill.
Perhaps you have some confusion about available fault currents. That's a different question, and a different topic. If so, go Googling, read some Wiki's, or find a textbook which deals with it. You'll get the hang of it.
My earlier answer is simple, and absolutely correct in nearly any instance. Manufacturers of the branch circuit panelboards, circuit breakers, and load devices have no say, no impact.
Gary I disagree. Manufacturers build the electrical equipment we design to, specify and use in the field. The manufacturers test them for durability, capacity and short circuit withstandiblity that we cannot violate without consequences. As an engineer it would be foolish not to consult manufacturers of the equipment they build.
Disagree with you on this, Gary.
I changed my mind, I may not have understood the original question, Gary. If it's supplementary OC protection, it really doesn't matter now, does it? You are correct, it is too simple, provided the OCPD is properly rated for the available fault current.
I would like to thank everyone for contributing to the discussion. I am afraid that indeed -- a mountain was made out of a molehill. According to my source --- although I don't understand the logic -- only the OCPD protecting the branch circuit has to have an interrupting rating which is capable of interrupting the available fault current. Even though the supplemental fuse is actually connected in the branch circuit, it is still supplemental.
I got confused when you said, "use an inline fuse to protect the device" which somehow made me think this was a question about equipment. Any equipment must be rated for the available fault current at the line terminals of the equipment, was my point. I apologize to Gary that I take part in such a simple and unknowledgeable forum (which I don't believe for a second), and that I can make a mistake.
Yes. It is perplexing to me. The fuse protecting the equipment is actually located in the branch circuit. However, from what I have been told, it is a supplemental device so outside the jurisdiction of the NEC.
Correct, out with the NEC in with NFPA 79, Chapter 7.
{7.2.4.1} … Tapped … in accordance with {7.2.4.2}. Such a tapped control circuit shall not be considered to be a branch circuit … protected by .. supplementary … OCPD.
Bare in mind, the Wire still needs proper Fusing or it becomes the Fuse Link. Example; an in-line 5A (Littlefuse). Wire size to the Fuse sized per the previous OCPD per the Table 12.5.1, after the 5A Fuse cannot be smaller than 20 AWG. Some of the ways around Sensor Cables and Devices - they are all ready self-protected or current-limited.
If you are dealing with branch circuits using no 12AWG conductors short circuit withstand ratings should not be an issue. But its important to work through the math in determining the available fault current anywhere in the AC system.
I don't think there is any way to get 10000amps to a device connected to a 20Amp circuit breaker in a panel having more than 10000amps available. Its the circuit breaker you want to be concerned about not the device at the end of a branch circuit. The wire alone between the circuit breaker and the device should limit the fault current to a value under 5KAIC based on its impedance.
Yes the manufacturers information does matter when using overcurrent protection in that they create the current curves and these curves must be consulted when using them in a circuit. When using overcurrent protective devices in series with one another its critical to consult with the manufacturers data on let through current and clearing times. The manufacturer also has charts for series ratings.
Thank you. I read passages of NFPA 79 this morning, as well as literature published by UL and Rockwell on branch circuit and supplementary protection.
Robert
That is ok for a fuse but what about the circuit breaker? What do you get when you overlap the curves [ fuse and circuit breaker ] under fault conditions?
Thank you Robert for that explanation. You are correct in your graphical presentation. However, I might add that fault current for branch circuits are not as a rule considered in the real world of building design. Placing a 20A circuit breaker in series with a control power fuse of 5amps, for example, is not an issue and not worth the exercise of calculating fault current or determining the clearing time of overcurrent protective devices. My question was merely for clarity. The usual reason you would want to know the clearing times of circuit protective devices is to provide an electrical service that is selective under fault conditions. If there was a fault somewhere in the AC system you would not want to trip the main and take down the entire buildings electrical service if a 20A circuit breaker were to trip under fault conditions.
Selective coordination is an absolute must for hospitals where critical care is an issue. If we were dealing with control circuits critical enough to be concerned about opening under fault or overload conditions we would probably use a technology suitable for the purpose such as and IPS or UPS system.
Thank you. Very clear.
This question has been answered and OP was satisfied and said thanks.
Yes.
Be certain to place it on the load side of the branch circuit protection.