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Old 08-24-2010, 06:53 PM   #1
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How to test wiring insulation?


Hey guys,
The HVAC condenser blew up yesterday after calling an A/C Repair guy to fix it (it'd been tripping the breaker for 2 days.) The disconnect at the unit outside was melted, and the insulation was burned 2-3 inches back from the connection point. He replaced the disconnect and turned it back on, there was a popping sound, and the motor blew.

The repair guy told me it is due bad wiring in the ceiling. I've a couple years experience as an electricians hand, so I'm comfortable working with Electrical, but never had need to test the wiring insulation. I've tested the line and it is getting 119 volts per leg with no variance without a load. I'd like now, to test the insulation so I can be positive it is not the wiring at fault. I've got a digital clamp meter with a 500v insulation tester. It didn't come with instructions for the insulation tester. My questions is, how do I use this to test the insulation, and am I correct in thinking that the problem was with the old condenser and not the wiring?

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Old 08-24-2010, 07:04 PM   #2
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How to test wiring insulation?


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Originally Posted by Shea_Close View Post
1 the insulation was burned 2-3 inches back from the connection point.

2 bad wiring in the ceiling.
1 high contact resistance + 20 A = high temp connection = localized melting of the insulation
and
high contact resistance can equal burned out motor.

2 it may be bad but I doubt that is a logical conclusion from what you've posted.

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Old 08-24-2010, 07:10 PM   #3
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How to test wiring insulation?


So, are you saying the likely culprit is a bad connection at the contact points?
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Old 08-24-2010, 07:55 PM   #4
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How to test wiring insulation?


what kind of meter do you have?


I suspect the problem is with the unit and not the wiring. The melted disco tends to show overcurrent. The fact the breaker had been tripping for a couple days tends to show the compressor or at least the capacitor had gone bad and the unit was simply trying to start over and over again. The high draw on start up over and over again quite likely melted everything.
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Old 08-24-2010, 08:19 PM   #5
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How to test wiring insulation?


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Originally Posted by Shea_Close View Post
So, are you saying the likely culprit is a bad connection at the contact points?
"Contact" meaning the contact between two surfaces; a fuse and fuse clip, a screw and the wire being held down, etc.

Here's a good tutorial on this
http://www.connectorsupplier.com/tec...ce_1-23-07.htm

Practically, for an improperly installed wirenut I measured 0.5 vac across it with a current of 10 A through it. This is 50 milliohms of resistance appearing at the interface between both wire surfaces. Sure enough, one of the three wires had no score marks on it and it was not at all twisted like the other two wires.
A good install probably reads 10x less resistance than this but the wirenut manufs are not exactly tripping over each other to tell me what a passing value is.

A book on electrical testing told me that the voltage drop across the closed contacts of a relay should be less than 30 mV at rated current.
For a 1 A relay this passing value of resistance is less than 30 milliohms. For a 100 A relay this is less than 300 micro-ohms.

More surface area or more force pressing the surfaces together, or both, means lower contact resistance.

I think the problem with alum. wire was that corrosion caused increased contact resistance, which was possibly not remedied by screwing down the bolts more tightly. The cross sectional area of the wire was certainly capable of carrying the current.

Last edited by Yoyizit; 08-24-2010 at 08:23 PM.
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Old 08-24-2010, 08:25 PM   #6
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How to test wiring insulation?


Quote:
Originally Posted by Yoyizit View Post

Practically, for an improperly installed wirenut I measured 0.5 vac across it with a current of 10 A through it. This is 50 milliohms of resistance appearing at the interface between both wire surfaces. Sure enough, one of the three wires had no score marks on it and it was not at all twisted like the other two wires.
A good install probably reads 10x less resistance than this but the wirenut manufs are not exactly tripping over each other to tell me what a passing value is.

A book on electrical testing told me that the voltage drop across the closed contacts of a relay should be less than 30 mV at rated current.
For a 1 A relay this passing value of resistance is less than 30 milliohms. For a 100 A relay this is less than 300 micro-ohms.

More surface area or more force pressing the surfaces together, or both, means lower contact resistance.

I think the problem with alum. wire was that corrosion caused increased contact resistance, which was possibly not remedied by screwing down the bolts more tightly. The cross sectional area of the wire was certainly capable of carrying the current.
WTF good does this do for the OP, or ANY diy for that matter????
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Old 08-24-2010, 09:08 PM   #7
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How to test wiring insulation?


Yoyizit.,

I know you want to help here but please just do NOT throw some way over compaited items in here most case they are not really benifit to the DIY'er at all.

Most of them don't even have specal testing tools like some of the Electricians will have.

Just keep it simple and to the point that will do the task very fine without getting overboard on the matter.

Merci.
Marc
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Old 08-24-2010, 09:26 PM   #8
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How to test wiring insulation?


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Sorry to disturb you.

Please ignore all of my present posts and any that I may write in the future.
That's fine with me. Most of them are worthless to this DIY forum anyway.
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Old 08-24-2010, 10:37 PM   #9
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How to test wiring insulation?


Alright, I think I have the testing figured out... It's a digital meter, I disconnected both legs of the line, connected one lead of the meter to one leg of the line, and the other lead to the ground. I got a reading of 1900-2000 on either line. Problem now, is that I don't have the slightest clue as to what that means... any takers?
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Old 08-24-2010, 10:42 PM   #10
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How to test wiring insulation?


Quote:
Originally Posted by Shea_Close View Post
Alright, I think I have the testing figured out... It's a digital meter, I disconnected both legs of the line, connected one lead of the meter to one leg of the line, and the other lead to the ground. I got a reading of 1900-2000 on either line. Problem now, is that I don't have the slightest clue as to what that means... any takers?
Replace the cable and be done with it IMO the reading is little too low to be confrotable.

That what I will do in first place { I have megger tester which it will read much higher voltage and I can able tell if good or not }

Merci.
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Old 08-24-2010, 10:51 PM   #11
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How to test wiring insulation?


Quote:
Originally Posted by Shea_Close View Post
Hey guys,
The HVAC condenser blew up yesterday after calling an A/C Repair guy to fix it (it'd been tripping the breaker for 2 days.) The disconnect at the unit outside was melted, and the insulation was burned 2-3 inches back from the connection point. He replaced the disconnect and turned it back on, there was a popping sound, and the motor blew.

The repair guy told me it is due bad wiring in the ceiling.
Did he say why he came to this conclusion? More than likely the motor had a short in the windings. A connection in the disconnect was loose or had loosened up over tine and caused excessive heating at the terminations. If the insulation on your wiring was bad causing a short your overcurrent device in your panel would have tripped.

Has the condensor been changed yet?
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Old 08-24-2010, 11:04 PM   #12
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How to test wiring insulation?


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Replace the cable and be done with it IMO the reading is little too low to be confrotable.
What is a normal reading? At what point does it become dangerous? I plan on replacing most of the wiring in my house, but wasn't ready to do it yet, do you think it could wait, or is it pressing?


Quote:
Originally Posted by codeone
Did he say why he came to this conclusion? More than likely the motor had a short in the windings. A connection in the disconnect was loose or had loosened up over tine and caused excessive heating at the terminations. If the insulation on your wiring was bad causing a short your overcurrent device in your panel would have tripped.

Has the condensor been changed yet?
This guy was a kid, I don't think he has any clue what he was talking about, my guess is he wanted to shift blame away from his company and/or product. They installed it a couple of years ago, and it is still under warranty.

And of course it hasn't been changed yet. They showed up at 4:45 today to get the part that needed replacing, and the parts shop closed at 5:00. Thank lord we have a window unit in my bedroom. Me, the wife, and kids are cramped, but cool.
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Old 08-25-2010, 02:08 PM   #13
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How to test wiring insulation?


It is not about trying to get you off the forum. It is about how a lot of your posts are way out of the DIY skillset, overly complicated or not the safest procedures to follow. For example, wrapping a conductor 10x around an object and then measuring the length of the coil and then dividing by 10 to get a decimal value and then you would still need to find a wire size chart. You also advised someone that was getting shocked from their siding and instead of saying it needed to be addressed as a life safety hazard, you posted about how to test the voltage level.

Adding posts about needing to get your BS meter calibrated don't add a lot either.

If you feel that some of your posts are useless why would you take your time to post them?

There are a lot of people here trying to help. You can be one also.
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Old 08-25-2010, 04:55 PM   #14
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How to test wiring insulation?


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