Replacing Blown Fuse Prior to T/Sing Authorization 1

[polareng]

I am searching for the words, in the AEH, NEC, IEEE, etc., that give guidance that it is acceptable to replace a blown fuse once before commencing troubleshooting on the condition that the replaced fuse does not immediately blow following replacment.

I am being asked to show proof of this, and I know I read it before somewhere, I thought maybe in the American Electrician's Handbook, but now I am not finding it.

 

[dpc]

I seriously doubt you will find that in any standard. It is a time-honored practice, but a poor one.

 

[Zogzog]

I think this is what you are looking for, but it just says you cant. Now if you can prove it was an overload, not a fault, you get 1 free shot, but that really only applies to breakers that have indication of the type of trip, fuses dont tell you that.

OSHA 1910.334(b)(2)Re-closing circuits after protective device operation. After a circuit is de-energized by a circuit protective device, the circuit may not be manually reenergized until it has been determined that the equipment and circuit can be safely energized. The repetitive manual re-closing of circuit breakers or re-energizing circuits through replaced fuses is prohibited.

Note: When it can be determined from the design of the circuit and the over current devices involved that the automatic operation of a device was caused by an overload rather than a fault condition, no examination of the circuit or connected equipment is needed before the circuit is reenergized.

 

[davidbeach]

Note: When it can be determined from the design of the circuit and the over current devices involved that the automatic operation of a device was caused by an overload rather than a fault condition, no examination of the circuit or connected equipment is needed before the circuit is reenergized.

And a blown fuse is not an indication of an overload, but rather it is an indication of a fault.

 

[polareng]

Thanks all for the replies.

I was trying to capture the event that sometimes a fuse just blows out of the blue and there are no signs of symptoms for those who are questioning the cause of the fuse blowing. It is easy for me to understand this just happens occasionally (e.g., old/faulty fuse), but if I have supporting evidence (documentation) showing that this happens occasionally and that it is okay to replace one fuse in this event without extensive troubleshooting, I would have more of a case.

Although it does say, "the circuit may not be manually reenergized until it has been determined that the equipment and circuit can be safely energized." Which, tells me that if there is no visible damage or simple ground fault check of circuit produces no ground fault present then I should be able to perform a one time replacment of the fuse before performing more further in depth troubleshooting.

 

[dpc]

Fuses do sometimes just give up the ghost, especially the low ampere fuses. I think a good visual inspection and perhaps meggering the downstream circuit would be sufficient if nothing else seemed to explain the situation. Fuses that have seen fault current without completely melting can be damaged and trip for apparently no reason.

I would replace all three fuses.

Some of the "old" electricians around here may have better ideas.

 

[Zogzog]

I think you are trying to make it say what you want to hear. Sorry. Fuses to fail due to aging, thermal stresses, previous enevts, etc, but that is not an assumption you can make. Almost always a fuse blows because there is an unsafe condition in the circuit, here the name "protective device
"

 

[polareng]

Your'e right Zogzog. No hard feelings. I appreciate your candor.

It's not that I don't like/want to troubleshoot, but on the flip side, when production is shutdown due to downed equipment, sometimes you need to reproduce a blown fuse for those standerby lookyloos that have all the answers to prove that there really is a problem or all you hear is "maybe it's a bad fuse" type comments while you are trying to concentrate on troubleshooting something that is not readily apparent, if you know where I'm coming from.

As a troubleshooting engineer, I just would like to have the authorized lattitude is all.

 

[Zogzog]

I know exactly where you are coming from, and depending on the situation and equipment, agree with you somewhat, however, you wont find the documentation you are looking for.

I blow a fuse in my car, I just replace it. Or at my last house, historic home, had a fuse box, replaced those too. A 600V fuse in a CAT III system, no way, bust out the megger and test the circuit first.

 

[waross]

I agree with you completely, dpc. I have watched young electricians replace just one fuse several times in a few minutes, until all three had been replaced. Dual element fuses on motors have been known to go ahead of the overload relays on jam-ups. Typically, one fuse blows and two fuses are damaged.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[roydm]

I think you should at least look at the fuse to determine whether it blew from age/mild overload or a short.
If it blew from a mild overload how would you troubleshoot that? Once you get running again you can measure the load.
If it blew from a short try to find the problem before re-energising.

Roy

 

[Zogzog]

You have some majic fuses Roy? Never seen one that told me why it blew.

 

[LionelHutz]

You have to do some tests first. If you don't find a problem then you can try it again. The OSHA does not say the cause of the fault needs to be found but that you need to determine if it is safe to try it again. Of course, something like a ground on one of the conductors does not make it safe. Checking for grounds and doing an ohm check of the motor and visible check of everything else at least makes it reasonably safe to try again.

It's better to test than get to the point that I've seen in the past. Customer wanting us to pay for 12 400A motor fuses because they just kept replacing them until they ran out of fuses even though the starter had an obvious problem.

You can cut apart the sand filled fuses such as a Ferraz AJT fuse and somewhat see what the problem was. You can see more element heat damage in the middle from an overload compared to a short circuit which is fairly uniform. But, it's not guaranteed.

 

[roydm]

ZogZog,
No, my fuses are the same dumb fuses everyone else has. If you open it up it's quite obvious if it was a short (vapourized metal, nothing much left) as oposed to overload (small gap, discouloured element).
Try a test, using a couple of small fuses.
For the porcela1n fuses wrap in a piece of rag and wack it with a hammer
Roy

 

[Zogzog]

Sure you cut them in half you can get some idea, but isnt it easier to verifry the circuit is safe to re-energiz than cut open a fuse to make an educated guess at best?

 

[waross]

The times, they are a changin!
Used to be standard practice to refuse and often re-blow.
Did it for years. Now with the emphasis on anticipating and avoiding all any any possibilities, even if the net hazard is greater, I would at least require a quik ground tes and phase to phase test with a multi meter before refuseing.
When I see crews of scaffolders building 20 ft high structures to save an electrician two trips up a ladder, I have to wonder if the overall hazard has been reduced.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Zogzog]

Hey I dont write the rules, I just post em'.

 

[Skogsgurra]

I follow this discussion with great interest.

The 'tourists' with all the answers mentioned by polareng, the scaffolding mentioned by Bill and the hours of discussions following the opening of a fuse are all too well-known.

One thing that I would like to add is that size matters.

The kind of fuses we use in Europe (Diazed aka "Poppenkorken") are used in low ampere circuits and the practice here is to replace a blown fuse without searching for the reason why it blew.

Above around 20 or 25 A. Cartridge fuses are used (even if there are Diazed fuses up to 63 A). A blown cartridge fuse is seldom/never replaced before the downstream circuits have been checked. And never with the circuit energized.



Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...