Fuse is blown but still reading continuity and voltage

Alfont1120

Member
Location
Rehoboth, MA
I've ran into this issue a few times at my plant. We have a blown fuse, but we will still get continuity AND voltage. I know you can read through a coil or xfmr so I get why sometimes you will have continuity, but the voltage is throwing me off. We have two fuses protecting a 480 to 120V (250VA) control transformer. I have 280 to each of the two fuses, on both the line and load side. I even have 280 at the xfmr, but when I go leg to leg I have 0V. Because I've ran into this before, I pulled the fuses and re-checked them, and sure enough, they were no good. How can this happen?
 

kwired

Electron manager
Location
NE Nebraska
I've ran into this issue a few times at my plant. We have a blown fuse, but we will still get continuity AND voltage. I know you can read through a coil or xfmr so I get why sometimes you will have continuity, but the voltage is throwing me off. We have two fuses protecting a 480 to 120V (250VA) control transformer. I have 280 to each of the two fuses, on both the line and load side. I even have 280 at the xfmr, but when I go leg to leg I have 0V. Because I've ran into this before, I pulled the fuses and re-checked them, and sure enough, they were no good. How can this happen?
You get the continuity through the load but are forgetting that you have voltage from the good fuse that is trying to return to the blown fuse through that same continuity path you just mentioned.

Assume L1 is good and L2 is open.

Line side you have 277 volts from each to ground. Load side of fuses you have 277 from L1 to ground, but that circuit extends through the transformer and back to L2 load side where the circuit is open. There is no voltage drop across the transformer primary if there is no current - you just have a long wire wrapped around the tranformer core many times that is connected to L1 on one end and open circuited on the other end.

If you measured from load side of fuses with L2 blown you will not read any voltage between the two fuses - they are at same potential which is the L1 potential. Both will read same voltage to ground - the L1 potential to ground.

If you read from line side of L2 to load side of L2 you will read 480 volts because you are essentially measuring L1 to L2.
 

kwired

Electron manager
Location
NE Nebraska
I apologize, I typed that out wrong. BOTH fuses were blown. Your theory on why we would be getting voltage is dead-on if only one fuse was burnt out...
Are you using a low impedance meter to test? Otherwise you are either reading capacitively coupled voltage or maybe there is a fault to some other circuit in your control circuit. Is the 120 secondary somehow paralleled with some other control circuit and you are reverse feeding the control transformer? If it is out of phase with the second source it may explain why it is blowing fuses, if it is in phase it can be that way indefinitely without you noticing.
 

Alfont1120

Member
Location
Rehoboth, MA
Are you using a low impedance meter to test? Otherwise you are either reading capacitively coupled voltage or maybe there is a fault to some other circuit in your control circuit. Is the 120 secondary somehow paralleled with some other control circuit and you are reverse feeding the control transformer? If it is out of phase with the second source it may explain why it is blowing fuses, if it is in phase it can be that way indefinitely without you noticing.
I'm using a new fluke 375. I don't see how there could be any capacitive voltage due to what is in the cabinet. I was reading a steady 280VAC, and we are not back-feeding the transformer; all that is on the load side of that is a DC power supply, which I had disconnected to rule it out (at this point, there were no wires hooked up to the load side of the xfmr, and we had still had line voltage but no secondary). This is also the first time these fuses have been blown on this machine. We had a power outage the other day, so that explains why we might have blown them, but it still doesn't answer why I was reading voltage.
 

Alfont1120

Member
Location
Rehoboth, MA

kwired

Electron manager
Location
NE Nebraska
I'm thinking something along those lines. Maybe dual element fuses can give you a false reading if one of the elements (for example the short-circuit) is blown but not the overload?

http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/solution-center/technical_library/BUS_Ele_Tech_Lib_Fuses_Time_Delay.pdf
But the two elements are in series with one another and this is even shown on the page you linked to, so open either element and you open the circuit, there about had to be carbon tracing across whichever portion was open.

Was there a high voltage transient involved with the outage incident?
 

Alfont1120

Member
Location
Rehoboth, MA
But the two elements are in series with one another and this is even shown on the page you linked to, so open either element and you open the circuit, there about had to be carbon tracing across whichever portion was open.

Was there a high voltage transient involved with the outage incident?

These are ancient fuses, they actually don't say dual element on them, I was just trying to think outside the box. Maybe these were constructed a little differently??
It doesn't appear that there was a voltage spike, due to the fact no other machines were affected. Apparently someone crashed into a pole down the road.
 
This sounds like a thing we used to call 'hanging fuses'. On occasion, fuses would blow but when the cooled off they would show continuity. I was told the element in the fuse would just barely touch back across the part that blew and to a meter it would look like it was OK. But, when in a circuit under a load they would act like they were blown.

The most common ones I saw do this, IIRC, were FRS-R 30 fuses on 480 volt equipment, like electric holding furnaces.
 

kwired

Electron manager
Location
NE Nebraska
These are ancient fuses, they actually don't say dual element on them, I was just trying to think outside the box. Maybe these were constructed a little differently??
It doesn't appear that there was a voltage spike, due to the fact no other machines were affected. Apparently someone crashed into a pole down the road.
If the elements are not in series, then they are in parallel, if so you would have to blow both elements to open the circuit no matter which type of fault was encountered.
 

Alfont1120

Member
Location
Rehoboth, MA
This sounds like a thing we used to call 'hanging fuses'. On occasion, fuses would blow but when the cooled off they would show continuity. I was told the element in the fuse would just barely touch back across the part that blew and to a meter it would look like it was OK. But, when in a circuit under a load they would act like they were blown.

The most common ones I saw do this, IIRC, were FRS-R 30 fuses on 480 volt equipment, like electric holding furnaces.

This fuse is a BUSS, which it looks like is an older version of the FRS type, and it just so happens it was in a control cabinet on an electric furnace...Looks like you might of just hit the nail on the head.
 

kwired

Electron manager
Location
NE Nebraska
Measuring voltage across fuses while connected to load doesn't lie, no voltage fuse is good, full applied voltage fuse is blown. Anything else may mean other troubles are present.
 
This fuse is a BUSS, which it looks like is an older version of the FRS type, and it just so happens it was in a control cabinet on an electric furnace...Looks like you might of just hit the nail on the head.
When I was working in maintenance in an aluminum die cast foundry, one of the first things I was warned not to do was grab a fuse out of a pile (instead of a new box) and use it because it checked OK with a meter.

The best way to check fuses while still in a panel is under a full load. That's not always possible or practical. If you found the reason for the fuses being blown, there really isn't much sense playing with the blown ones other than plain curiosity. I was guilty of that.
 

Alfont1120

Member
Location
Rehoboth, MA
When I was working in maintenance in an aluminum die cast foundry, one of the first things I was warned not to do was grab a fuse out of a pile (instead of a new box) and use it because it checked OK with a meter.

The best way to check fuses while still in a panel is under a full load. That's not always possible or practical. If you found the reason for the fuses being blown, there really isn't much sense playing with the blown ones other than plain curiosity. I was guilty of that.

That's all this was, just curiosity. The problem has been addressed and the line is running fine, I just like to know how and why everything works the way it does. Thank you everyone for your input!
 
That's all this was, just curiosity. The problem has been addressed and the line is running fine, I just like to know how and why everything works the way it does. Thank you everyone for your input!
Take one of the blown fuses that give you a funny reading and dissect it. Cut rings near the ends and carefully slide the element out of the 'sand'. Remove the 'tube' and position the element halves kind of back together to see where the element fused open. If it shows a clean break, it probably fused partially back together and broke when you took the fuse apart and would be a suspected culprit of funky readings.

Just out of curiosity, are the fuses mounted horizontally or vertically?
 

ADub

Senior Member
Location
Iowa
This sounds like a thing we used to call 'hanging fuses'. On occasion, fuses would blow but when the cooled off they would show continuity. I was told the element in the fuse would just barely touch back across the part that blew and to a meter it would look like it was OK. But, when in a circuit under a load they would act like they were blown.

The most common ones I saw do this, IIRC, were FRS-R 30 fuses on 480 volt equipment, like electric holding furnaces.
Had this happen last month on a heat treat oven.


Sent from my iPhone using Tapatalk
 

Aleman

Senior Member
Location
Southern Ca, USA
I've seen this happen once. Fuse blew but still managed to have some kind of short. Enough to look ok on a meter but if you put any current on it it
would drop most of the voltage.
 
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