Megohmeter uses

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jute

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SO CAL
Hello, I'm interested in the uses of Megohmeters...I was ask to inspect the wiring in an older house that is wired in knob and tube to see if rewiring was necessary or not. I don't know what kind of test or test equipment would be used that would show that the wiring is acceptable or is in need of repairs or rewiring. There's alot of homes wired in knob and tube and I'd like to perform a visual inspection of the wiring system for loose connections, brital insulation, bare wire, splices, circuit tapped into and wires hanging in air with electrical tape etc..) I'd also like to be able to test this and show the HO that the wiring is in good shape or bad shape and in need of rewiring....(I'm thinking a megohmeter??). I found a "Knob and Tube Wiring Safety Inspection Report", with this inpection report, a basic visual test and test of some sort ... I'm using this to try and sell residential rewiring...Any suggestions???Thanks, JB
 

Dennis Alwon

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Location
Chapel Hill, NC
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Retired Electrical Contractor
Others will chime in on the use of a megger meter.

There are other ways to sell a rewire from K&T to new wiring. The simply fact that some insurance companies won't insure the house or they will charge a premium if there is K&T wiring.

There is also the safety issue with K&T--- not having an EGC. I would approach it from that side rather than a meg.
 

mdshunk

Senior Member
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Right here.
I highly doubt a megger would be of any value in evaluating knob and tube wiring. I say this mostly because of the fact that the conductors were mounted so far apart. They only merge at each device, and there's no ground to tell if they're shorted to the box. I condemn knob and tube on age, visual appearance, texture in the hand, and by evidence of overfusing. The International Existing Building Code and The International Property Maintenance Code both contain text that requires you to condemn wiring and fuse panels that show any evidence of overfusing.

If you're looking for an instrument that would be of some value in evaluating vintage wiring systems, a DLRO would be what you'd want to look into. You're into the many 1000's of dollars, right off the bat, for one of those.
 

jute

Senior Member
Location
SO CAL
Dennis Alwon said:
There is also the safety issue with K&T--- not having an EGC. I would approach it from that side rather than a meg.

Thanks for the quick reply and this is exactly what I'm looking for... I know about the insurance companies not wanting to insure HO due to existing knob and tube but what can be said about the safety or hazards (that a HO would understand) as far as no EGC besides the fact that isn't one...Thanks, JB
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
jute said:
I know about the insurance companies not wanting to insure HO due to existing knob and tube but what can be said about the safety or hazards (that a HO would understand) as far as no EGC besides the fact that isn't one...Thanks, JB

Explain the purpose of the EGC. Tell them of the shock that can occur and how they cannot plug in their 3 prong appliances. Educate them that the adapters for their 3 prong appliance do not protect them or the appliance.

Remind them that with today's consumption of power the circuits will probably be overload. I suspect the fuses in the panel-- if they have fuses may be oversized.

Don't overdo it but simply explain and let them decide.
 

jute

Senior Member
Location
SO CAL
Thanks for the help and info on this, I think I need to take some training classes on the megger, but this info will definately help in hopefully getting some rewire jobs...Thanks again and have a great weekend, JB
 

catchtwentytwo

Senior Member
jute said:
Thanks for the help and info on this, I think I need to take some training classes on the megger, but this info will definately help in hopefully getting some rewire jobs...Thanks again and have a great weekend, JB
Here's a good way to start learning about insulation testing (from Megger) A Stitch in Time: The Complete Guide to Electrical Insulation Testing . The booklet is small enough to easily carry in a tool box. And it is only $7.
 

brian john

Senior Member
Location
Leesburg, VA
A megger will show if the insulators and associated wiring at the insulators is breaking down, but IMO the best tester for this type of installation is common sense, your eyes and maybe a camera
 

jute

Senior Member
Location
SO CAL
brian john said:
A megger will show if the insulators and associated wiring at the insulators is breaking down, but IMO the best tester for this type of installation is common sense, your eyes and maybe a camera

I have a little common sense, my eyes are bad but I use my camera all the time for quotes, tracking down parts and ???
I'm doing a knob and tube rewire job and 70% if not more houses in the area are in knob and tube...I need this info to put on a sales flyer to try and get more work so the info from here and a couple of pics should do it...Thanks again for the replys......JB
PS. Thanks catchtwentytwo, I ordered >A Stitch in Time: The Complete Guide to Electrical Insulation Testing. Should be arriving shortly...
 

HighWirey

Senior Member
"Hello, I'm interested in the uses of Megohmeters"

Originally Posted by mdshunk
"I highly doubt a megger would be of any value in evaluating knob and tube wiring"

True, in order to establish insulation integrity or isolation, the item under test must have something with which to reference: (ground, neighboring conductors, wet wood, water, dust . . .) I suspect that K&T conductors are fairly well isolated from each other and from the world, except at an outlet box. At a box you have no ground for reference, and you are back to square one. Mentioned by others here, a good visual, followed by a customer demo, sounds like your best bet.

As I remember, the AVO/Biddle pamphlet 'A Stitch in Time' (p/n AVTM21-P8B) is a simple primer on megohmeter technique. Don't think it covers the specific megohmeter uses you are seeking. Someone will fill us in.

Originally Posted by HotWire367
"Do you guys have a recommendation on a decent megger in the 300-500 dollar range?"

Like most other test equipment, there are used, new, analog and digital units. All are fairly $pendy. Analogs were adequate in my orbit, and were very robust. I used 500/1,000 volt meggers for years, but job specs changed over time (higher applied voltage is now required by some specs). I lucked out and found this near new Biddle BM11, 5kv megger at a hamfest for $200. Another $150 for check out and calibration, good to go. Still humming along on its origional rechargeable batteries! This older megger is advertised today for $5260, may be cheaper on flee bay. I believe the 5kv rating accounts for the price disparity.

Other megohmeter uses . . .

Now-a-days, since I am under utilized, Mr Biddle and I go out on the river, drop his long leads in the water, apply one or two 'brrzzizzs' and presto, instant reds (thats all I can do since they took my dynamite). It's faster, cheaper, better, and quieter anyway (what a winning combo) and . . . GF thinks I spent all morning fishin' my heart out ;)

Best Wishes
 

brian john

Senior Member
Location
Leesburg, VA
True, in order to establish insulation integrity or isolation, the item under test must have something with which to reference: (ground, neighboring conductors, wet wood, water, dust . . .) I suspect that K&T conductors are fairly well isolated from each other and from the world, except at an outlet box. At a box you have no ground for reference, and you are back to square one. Mentioned by others here, a good visual, followed by a customer demo, sounds like your best bet.

While I tend to agree with you on this, insulators break down all the time, why else would bus in a switchboard have deterioating readings as years pass?
 
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