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My Supco M500 Insulation Tester/Electronic Megohmmeter “usually” works pretty good at finding low voltage shorts.
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__________________________________________________ _______________________
“Experience is a hard teacher because she gives the test first, the lesson afterwards" ~ Vernon Law
"It's what you learn after you know it all that counts." ~ John Wooden
"When the teachers become unteachable we're all in trouble" ~ Mr. Bill
"Remember "Pro" is only a name, it's not always a mindset determined to do everything correctly" ~ Mr. Bill
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Intermittent low voltage shorts are some of the most fun you will ever have in this trade. If you do not learn to laugh at it, it will drive you nuts!
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Originally Posted by
Mr Bill
My Supco M500 Insulation Tester/Electronic Megohmmeter “usually” works pretty good at finding low voltage shorts.
Sent from my iPhone using Tapatalk
That is interesting. Please tell how you use it.
I like that idea. Why did I not think of that.
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Low voltage fuse blowing
Last time I used it, I used it to find a low voltage short under a mobile home. I clipped one of the alligator clips “that’s what us old school folks call them” on the ground inside the condenser service panel, and with the unit off, and with the other clip, I checked both low voltage wires, and it was reading short. I didn’t even crawl under the trailer to verify, I just pulled another wire with the existing wire, problem solved.
This system was popping a fuse about every 2-3 days, 2 service companies out couldn’t figure it out, probably didn’t have a meter, or didn’t want to crawl under the trailer.
Great little tool for around $100.00
__________________________________________________ _______________________
“Experience is a hard teacher because she gives the test first, the lesson afterwards" ~ Vernon Law
"It's what you learn after you know it all that counts." ~ John Wooden
"When the teachers become unteachable we're all in trouble" ~ Mr. Bill
"Remember "Pro" is only a name, it's not always a mindset determined to do everything correctly" ~ Mr. Bill
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Do you have the ability to run a temporary wire from the inside to the outside machine? I had one that I finally did that on my 3rd trip out and somewhere that wire was no good at certain times for whatever reason. In fact that was 2 years ago and to my knowledge they never had us come and actually replace the wire. I’m sure it’s still running down a hallway through a bedroom and out the corner of a window to the condenser.
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Originally Posted by
Mr Bill
Last time I used it, I used it to find a low voltage short under a mobile home. I clipped one of the alligator clips “that’s what us old school folks call them” on the ground inside the condenser service panel, and with the unit off, and with the other clip, I checked both low voltage wires, and it was reading short. I didn’t even crawl under the trailer to verify, I just pulled another wire with the existing wire, problem solved.
This system was popping a fuse about every 2-3 days, 2 service companies out couldn’t figure it out, probably didn’t have a meter, or didn’t want to crawl under the trailer.
Sent from my iPhone using Tapatalk
Thanks Bill,
I was pretty much thinking that would be the procedure.
Not sure how that would work with an intermittent short. You pretty much have to catch it in the act of shorting out.
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Originally Posted by
motojo646
This works but is a bit clumsy. I've had to use this method on a few heat pumps. I've used 1 amp fuses instead of 3
I'm now not sure if this method is reliable assuming that you will still need to have a fuse on the control board to protect the board itself. So if you have both the branch fuses and the main fuse in place during a short can you predict which one will blow first? That might then provide misleading results. Or am I missing something.
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Originally Posted by
szw21
I'm now not sure if this method is reliable assuming that you will still need to have a fuse on the control board to protect the board itself. So if you have both the branch fuses and the main fuse in place during a short can you predict which one will blow first? That might then provide misleading results. Or am I missing something.
Put a 3 amp on the branch circuits and a 5 amp on the board. You could probably even get away with one amp on them branches but you don't want false fuse blows sending you on wild goose chases.
Sent from the Okie state usin Tapatalk
"Is this before or after you fired the parts cannon at it?" - senior tech
"Is this before or after you fired the parts cannon at it?" - senior tech
I'm tired of these mediocre "semi flammable" refrigerants. If we're going to do it let's do it right.
Unless we change direction we are likely to end up where we are going.
"It's not new, it's better than new!" Maru.
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Originally Posted by
Averagetech37
Had one helluva time in a intermittent short like this finally after a few hours staring at an operating furnace I heard footsteps above me in hallway and saw my furnace glitch. Turns out tstat wire was ran tight between ductwork and floor joist and once someone walked in the floor joist it sank enuff to pinch the wire and short…… gotta love intermittently shorts
Since this is a 3 year old home a construction related error could well be a possibly. On the plus side the attic where the furnace is located was nice and cool now that radiant barriers/cool roofs are code. I was pretty comfortable in the middle of the day helped by my small Milwaukee fan!
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Originally Posted by
R600a
Put a 3 amp on the branch circuits and a 5 amp on the board. You could probably even get away with one amp on them branches but you don't want false fuse blows sending you on wild goose chases.
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"Is this before or after you fired the parts cannon at it?" - senior tech
But the board could be the source of the problem also correct? Is it ok to have a 5A on the board. I guess I don't know if anything on the board will fail catastrophically with a larger fuse.
Would 1A work for the condenser contactor? I measured about 15 ohms coil resistance so a dc current draw will be around 1.6A. Of course since this is ac for a more accurate result we would need to know the inductance and capacitance of the coil. Not sure if that will affect the current draw much without doing the calculation.
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Originally Posted by
joemach
Thanks Bill,
I was pretty much thinking that would be the procedure.
Not sure how that would work with an intermittent short. You pretty much have to catch it in the act of shorting out.
I wonder if the short is not quite enough to blow a fuse all the time time but intermittently but maybe enough to register on the tool.
I was debating adding this to my toolkit. However I read multiple old posts on this and several that said this was a cheap tool and not very accurate. I was thinking about getting a better Megger but then again never really needed it. I might reconsider getting one.
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Originally Posted by
szw21
But the board could be the source of the problem also correct? Is it ok to have a 5A on the board. I guess I don't know if anything on the board will fail catastrophically with a larger fuse.
Would 1A work for the condenser contactor? I measured about 15 ohms coil resistance so a dc current draw will be around 1.6A. Of course since this is ac for a more accurate result we would need to know the inductance and capacitance of the coil. Not sure if that will affect the current draw much without doing the calculation.
I use only 5 amp fuses and I don't know of anywhere that it ever caused a problem.
Sent from the Okie state usin Tapatalk
"Is this before or after you fired the parts cannon at it?" - senior tech
"Is this before or after you fired the parts cannon at it?" - senior tech
I'm tired of these mediocre "semi flammable" refrigerants. If we're going to do it let's do it right.
Unless we change direction we are likely to end up where we are going.
"It's not new, it's better than new!" Maru.
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Another good thing to do is make a loop of 10 wraps of 14 gauge wire with a terminal on either end and connect that in series with any portion of the low voltage system that you want to check the amperage on. Just put your amp clamp through the loop and it will read one decimal place higher than the actual amperage.
Sent from the Okie state usin Tapatalk
"Is this before or after you fired the parts cannon at it?" - senior tech
"Is this before or after you fired the parts cannon at it?" - senior tech
I'm tired of these mediocre "semi flammable" refrigerants. If we're going to do it let's do it right.
Unless we change direction we are likely to end up where we are going.
"It's not new, it's better than new!" Maru.
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Originally Posted by
R600a
I use only 5 amp fuses and I don't know of anywhere that it ever caused a problem.
Sent from the Okie state usin Tapatalk
"Is this before or after you fired the parts cannon at it?" - senior tech
Yes I guess 5A should be ok.
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Originally Posted by
R600a
Another good thing to do is make a loop of 10 wraps of 14 gauge wire with a terminal on either end and connect that in series with any portion of the low voltage system that you want to check the amperage on. Just put your amp clamp through the loop and it will read one decimal place higher than the actual amperage.
Sent from the Okie state usin Tapatalk
"Is this before or after you fired the parts cannon at it?" - senior tech
Good idea - never thought of that.
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Originally Posted by
szw21
Good idea - never thought of that.
I learned it from @TechmanTerry
Sent from the Okie state usin Tapatalk
"Is this before or after you fired the parts cannon at it?" - senior tech
"Is this before or after you fired the parts cannon at it?" - senior tech
I'm tired of these mediocre "semi flammable" refrigerants. If we're going to do it let's do it right.
Unless we change direction we are likely to end up where we are going.
"It's not new, it's better than new!" Maru.
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Originally Posted by
szw21
I wonder if the short is not quite enough to blow a fuse all the time time but intermittently but maybe enough to register on the tool.
I was debating adding this to my toolkit. However I read multiple old posts on this and several that said this was a cheap tool and not very accurate. I was thinking about getting a better Megger but then again never really needed it. I might reconsider getting one.
As for as the tool/meter itself. IMO the quality "it's all relative" the meter works for me, and for something I don't use but maybe twice a year, I'm personally not spending megabucks on one, and I'm not even a frugal person. You asked "I wonder if the short is not quite enough to blow a fuse all the time time but intermittently but maybe enough to register on the tool". That is exactly what this tool does, this is how a Megohmmeter works, if you reread most post, this mobile home the fuse would pop around every couple to 3 days. 
__________________________________________________ _______________________
“Experience is a hard teacher because she gives the test first, the lesson afterwards" ~ Vernon Law
"It's what you learn after you know it all that counts." ~ John Wooden
"When the teachers become unteachable we're all in trouble" ~ Mr. Bill
"Remember "Pro" is only a name, it's not always a mindset determined to do everything correctly" ~ Mr. Bill
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Originally Posted by
szw21
I was debating adding this to my toolkit. However I read multiple old posts on this and several that said this was a cheap tool and not very accurate. I was thinking about getting a better Megger but then again never really needed it. I might reconsider getting one.
This one might be better, knock your lights out.
__________________________________________________ _______________________
“Experience is a hard teacher because she gives the test first, the lesson afterwards" ~ Vernon Law
"It's what you learn after you know it all that counts." ~ John Wooden
"When the teachers become unteachable we're all in trouble" ~ Mr. Bill
"Remember "Pro" is only a name, it's not always a mindset determined to do everything correctly" ~ Mr. Bill
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WOW 
. Someone is really proud of their stuff or you really do get what you pay for 🤔
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Where was the last place I used my megger dammit !!
That would be me 😆
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