# Tracing a short / ground



## gpop (May 14, 2018)

Vodcoffie said:


> Good evening,
> To start off I work with are furnaces, to say the least it's very hot.
> A few days ago I was confronted with a short in a control circuit. Circuit on consists of a couple of limit switches and the normal start/stop circuit.
> 
> ...


Its low voltage (12-120v) so a megger can do a lot of damage to a control panel. Also most control panels have a neutral that is grounded so the megger isnt the best tool in the box.

A good ohm meter will tell you more than a megger and as you gain experience you will be able to tell whats hooked up just by the ohm readings.

Ohm a wire thats connected to the starter and you will see x amount of ohms. Meg it and it will show practically a dead short. 

The only time you should consider the megger is if you have a temperamental problem and you believe its on a wire. Then you will disconnect both ends and meg it.
If you are lucky most faults are dead shorts and with in a few minutes a meter set on ohms will get you to the problem.


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## Vodcoffie (Jun 26, 2018)

Thanks for the quick reply! 

That's is precisely what I was looking for, a temperamental problem over a long pipe that is under high heat (possible melted jackets). With all ends disconnected. 
I really should have said that in the previous post, my apologies.

I really appreciate the information, thank you.


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## Vodcoffie (Jun 26, 2018)

I should also say I used the megger on the 250v setting only. To prevent unnecessary leakage.


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## paulengr (Oct 8, 2017)

600 V insulation is tested to 2200 V.

In hot areas remember THHN and XHHW melts at 140 F. Going to fire alarm TPE gets you to 200-250. Virgin Teflon is just shy of 400. Above that the connectors are a problem and wire is high nickel but TPC for instance has a 2800 F cable.

I’m a former iron foundry guy.


Sent from my iPhone using Tapatalk


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## gpop (May 14, 2018)

a megger is good for reading as low as 100,000 ohms.

Most control circuits are 120v @ 3 amp (modern controls are 24v) so you would need a short less than 40 ohms or less to blow the fuse. That's easy to find with a regular meter with out disconnecting the wires. (like i said with experience you will learn to spot things like transformers that will give you a low ohm reading and take them in to account)

You disconnected the wires before testing so its all good in this instance. Had i seen you do this i might have told you that you did it wrong (even if it was right) because i know that one day you are going to be in a plc cabinet and the megger isn't the first tool you should be reaching for. 

Now if you had said that you tested with a ohm meter and couldn't find the fault so you disconnect the wires and switched to the meg then you would have got a "that a boy"


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## Vodcoffie (Jun 26, 2018)

gpop said:


> a megger is good for reading as low as 100,000 ohms.
> 
> Most control circuits are 120v @ 3 amp (modern controls are 24v) so you would need a short less than 40 ohms or less to blow the fuse. That's easy to find with a regular meter with out disconnecting the wires. (like i said with experience you will learn to spot things like transformers that will give you a low ohm reading and take them in to account)
> 
> ...


That makes total sense, again thanks for the help!


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## Cow (Jan 16, 2008)

Vodcoffie said:


> consists of a couple of limit switches and the normal start/stop circuit.





gpop said:


> Its low voltage (12-120v) so a megger can do a lot of damage to a control panel. Also most control panels have a neutral that is grounded so the megger isnt the best tool in the box.
> 
> A good ohm meter will tell you more than a megger and as you gain experience you will be able to tell whats hooked up just by the ohm readings.
> .



Different strokes. I use a Fluke 1507 megger on control circuits without an issue. It might take me an extra minute to set it up and get the leads connected, but that doesn't matter to me.



If there is any question that I may have left something hooked up by accident, which isn't very often, I'll test at 50v until I narrow it down. 



Otherwise for something simple like the OP mentioned, which sounds to be a few basic limit switches, isolate the wires, hit it with 250vdc, and be done with it. An ohm meter isn't going to tell you that you found the short but a few feet away in another conduit another wire is rubbed through and about to be a ground fault.


But I do agree with someone using some common sense when operating a megger. If a guy is just running around zapping everything with 1000vdc, he better carry a lot of spare parts.


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## MDShunk (Jan 7, 2007)

Seems like commercial boiler limit switch wiring is almost an every 15-20 year PM replacement. Or, maybe it should be. By the time you find a short, the rest isn't far behind. Few boilers are so large or complex that the wiring for the cutoffs can't be replaced in an hour or so.


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## splatz (May 23, 2015)

Cow said:


> If there is any question that I may have left something hooked up by accident, which isn't very often, I'll test at 50v until I narrow it down.


Do you start at 50V and work your way up?


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## Cow (Jan 16, 2008)

splatz said:


> Do you start at 50V and work your way up?



Residential-I usually start at 50v and work my way up since I can't see where the wiring is going 100%


Everything else, for instance 480v motors and wiring, feeders, etc I hit it with 1000vdc on the first pass.


When you're megging you just have to remember you're okay to meg h-gr, n-gr without any issues. It's when you send test voltage between h-n and across an unknown load that you'll start burning stuff up if you failed to disconnect and isolate the load first. Those are the tests I'll start off with 50vdc first as a feeler to see if I might of missed anything still connected.


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## gpop (May 14, 2018)

Personally if i had come across the same problem i would have tested the fuse with the t5 then jump across the blown fuse with a wiggy , unhook a few wires and test again until i find the one that clears it. Seen if there was a spare that tested good then made a work order to repull it later along with everything in the pipe including the ones that tested good. Not the correct way to do it but a quick on the fly fix that avoids having to go get the meg And a multi meter from the shop.
If someone asks me the correct way then it the same as my first post


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## Cow (Jan 16, 2008)

gpop said:


> If someone asks me the correct way then it the same as my first post



If using an ohm meter is the only correct way in your mind to troubleshoot a control circuit like this right from the get go, then we'll agree to disagree.


I know when I jump out of the truck with a service call like this on my mind, my T+ Pro is already in my bag and I throw the megger in with it before I walk into the plant or wherever.


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## gnuuser (Jan 13, 2013)

providing the electronics are disconnected first (if it suspected a wire issue) using a megger is ok 
quite often a multimeter may not indicate an intermittent fault.
steps to follow multimeter first, disconnect conductors and meg if needed if not able to obtain satisfactory results with the multimeter.

using a wiggy for general troubleshooting (as long as voltage is present) is a quick way find open circuits and blown fuses, BUT should never be used downstream of a vfd or plc/slc triac as you risk burning out the hardware.
also there is generally a 15 second time limit for the wiggy to prevent its being burned out.

control circuits are usually not megged unless the run is exceptionally long!
most often the fastest method for correcting a possible wire issue is to pull a new run through the suspected area.
not what many want to hear but were talking industrial and manufacturing 
and excessive downtime cost them a lot of money.

meggers are great for motor issues but a lot of the time its an issue of common sense if the motor is hot and smells like burnt $#!t on a stick its a no brainer!


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