possible stupid question about fuses

[shawn73]

I have been an industrial electrician for almost 5 years and I am also working on an electrical degree. There is an engineer at my plant that I always ask questions and 99.9% of the time his information is valid. We had a discussion the other day in which I don't agree with him, or perhaps I truely don't understand. Here is my question...

I was told that fuses are for short circuit protection (phase to phase) NOT short to ground protection (phase to ground. Is it true that a fuse in some situations will not blow even though it has a solid ground? I can understand if you have a very slight ground that there may not be enough current to exceed the protective rating of the fuse. My head tells me if there is a difference of potential between 2 wires (ground always being 0) and they are connected without a load, current will increase until it excedes the rating of the protective fuse. Am I correct or did I miss out on some sort of theory??? Does transformer winding configuration (Delta or Wye) make some sort of difference? Thanks for all your help.

 

[rattus]

Re: possible stupid question about fuses

If he is talking about an ungrounded system, neither a fuse nor a breaker would blow on a phase to ground fault because there is no complete loop. But, if the current limit of the fuse or breaker is exceeded for any reason, you can expect it to do its duty.

 

[avr]

Re: possible stupid question about fuses

Well, a main breaker will protect from phase to phase or phase to ground and large overloads. You are allowed to use fuses in a main disconnect to a service. A fuse in a motor disconnect will blow if the load is grounded to the housing of the motor even though it is meant for overload protection. On my master test I remember that in the motor calculation I had to size the FUSES for the FEEDER SHORT CIRCUIT PROTECTION. My opinion is that the fuse will serve both purposes.

 

[Ed MacLaren]

Re: possible stupid question about fuses

I was told that fuses are for short circuit protection (phase to phase) NOT short to ground protection

Your difference of opinion with your engineer friend may be a matter of terminology.

In general, fuses and breakers will both provide acceptable overcurrent protection when used as intended.
Each has certain inherent advantages and disadvantages.

Both will open a circuit reliably in the event of a short to the grounded conductor of the system, or to an equipment/conductor enclosure if it is properly bonded to the grounded conductor of the system.

Neither will open reliably in the event of a short to an equipment/conductor enclosure that is not properly bonded, that is, if it is connected only to the earth.

Ed

[ March 06, 2005, 12:42 PM: Message edited by: Ed MacLaren ]

 

[avr]

Re: possible stupid question about fuses

Rattus, unless your equipment is 100% PVC then you will have a ground for protection of the metal parts. If there is no ground then this persons question is not even relevant. (No disrespect intended) Yes, phase to phase or phase to ground, I also believe the fuse will serve its purpose.

 

[avr]

Re: possible stupid question about fuses

A short circuit is when the currant basically falls short of the load. Phase to phase or phase to ground, the currant never makes it to the load.

 

[hurk27]

Re: possible stupid question about fuses

AVR on a grounded service system that would be true as the fault path would have a return back to source (the service transformer)
But I believe if he asked his EE if the service in their building is grounded he would get a NO response. This is allowed in industrial applications where down time from ground faults can disrupt production or cause a safety problem for workers. A fault would only occur if a second phase was to fault to ground. It should have a ground fault monitor on the service to allow it to be repaired before the second ground fault occurs.

See 250.21

 

[hurk27]

Re: possible stupid question about fuses

You guys type fast.

I should add that also he could have a hi impedance grounding also and the same result would happen unless the fault was to exceed the service GFP. But in an ungrounded system equipment grounding is still connected to the grounding electrode but there is no bond (MJB) to the service grounded conductor. These service are always a delta service wether 480 or 240 volt.

 

[avr]

Re: possible stupid question about fuses

hurk27, good to know. My industrial is limited. I will remember that. So possibly in his application the ee is correct but not in all applications. Correct?

 

[rattus]

Re: possible stupid question about fuses

avr, EEs are always right, even when they are wrong, and especially when they disagree with other EEs!

If the equipment is grounded, but the service is not, a fault might give you a corner grounded delta service without blowing a fuse.

[ March 06, 2005, 01:04 PM: Message edited by: rattus ]

 

[avr]

Re: possible stupid question about fuses

OK rattus. Note taken. Thank you for your input.

 

[hurk27]

Re: possible stupid question about fuses

By Avr: So possibly in his application the ee is correct but not in all applications. Correct?

Yes correct.
As in any grounded service you would have a fault path back to the transformer.
First rule is to always know what type of service you have in a building and what voltages you can expect before working on any system.

 

[jim dungar]

Re: possible stupid question about fuses

Fuses melt (blow) and breakers trip when too much current flows through the circuit. It makes no difference if the current is line-line, line to ground, line to neutral.

You need to ask your engineer what is really being talked about. For example, it is very common to not use fusible switches for service entrance equipment when ground fault protection is required which could be misinterpreted to mean fuses are not used for phase to ground currents.

 

[safeguy]

Re: possible stupid question about fuses

Generally speaking, whether a fuse or breaker is used, they will provide protection, when the sizing is correct, for any circuit grounded or ungrounded, from phase to phase or phase to ground.
The fuse will blow and the breaker will open...
The only difference is that " only " the fuse is FAIL SAFE...while the breaker could malfunction and not trip...IT HAS HAPPENED MORE THAN ONCE...!

 

[iwire]

Re: possible stupid question about fuses

Originally posted by safeguy:
Generally speaking, whether a fuse or breaker is used, they will provide protection, when the sizing is correct, for any circuit grounded or ungrounded, from phase to phase or phase to ground.

Safeguy, On an ungrounded system as Wayne has pointed out neither fuses or breakers will operate on ground faults.

 

[jim dungar]

Re: possible stupid question about fuses

Bob,
If the system is ungrounded then the first phase to ground connection is not a "fault". No standard OCPD will protect against something that doesn't exist.

 

[ray94553]

Re: possible stupid question about fuses

Just to point out, safe guy said "The only difference is that " only " the fuse is FAIL SAFE" and that is true, however I have had fuses that, with the load side disconnects open (to prevent any feed back from other sources) indicated that they were "Blown", but still had voltage present on the load side.
This is because it was tracking on a hi resistance current path inside the fuse barrel.
There isn't enough current available to operate the circuit but there is voltage.
And that's a good reason to check for the presence of voltage regardless of what a blown fuse indicator might indicate.
It also makes for a fun trouble shoot in the wee hours of the morning when you aren't prepared for the gremlins.

 

[rpmlube]

Re: possible stupid question about fuses

Shawn, if you go to www.bussmann.com they have a lot of good info on fuses.