megging wire

[sokkerdude]

Is there a written standard accept procedure for megging wire ? And what is acceptable reading for THHN?
I have a owner rep that is pissed off at my electrician on the job. He is wanting every wire megged. Which is not a problem, but he want accept our procedure. He is wanting to fill the conduits with water and meg the each wire for 2 minutes. Thank you

 

[don_resqcapt19]

He is wanting to fill the conduits with water and meg the each wire for 2 minutes.

Actually I like that idea. I have never seen it done, but I like it. If the conduit is dry you can have bare copper as long as it is not touching the metal conduit and pass a megger test with flying colors.
Don

 

[AE-29]

I believe it's 1000 ohms per volt, but I usually question anything under .5meg. With newly installed conductors you should get well over 100 megs since there should not be much wear. Also thhn is not for wet use, so I wouldn't unnecessarily fill a conduit with water .

 

[don_resqcapt19]

Also thhn is not for wet use, so I wouldn't unnecessarily fill a conduit with water.

True, but I have never seen THHN that was not also marked THWN. It may exist, but I have never found any.
Don

 

[petersonra]

I have never heard of anyone filling the conduits with water first, but it might reveal some insulation flaws that might not be caught otherwise. Personally, I question whether the regular use of megger testing is cost effective. I think it is a valuable tool for correcting problems down the road, or inspecting installations you might have doubts about, but there are a lot of things that can go wrong with an electrical installation and megging the wires only catches a very few of them.

Two minutes per wire sounds like it could end up taking a long time.

 

[Rockyd]

Let me give you a standard that I've used for years.

You need a set of radios

A good push button megger, up to a 1000v for 600v wire.

Ensure that both ends are free...your megging the wire only.

All wires should read 50+ megaohms, at a one minute test.

The water is somebody's pipe dream...seen wet cables bounce around 3 to 5 megaohms.

Most large cables should climb out over 2 gigaohms,heading towards infinity, after intial load up of cable/wire.

Make sure to drain cables if tester is not self draining!


Can't imagine getting hauled on the carpet, unless there is a lot of shafe around the conduit openings...then megging needs to be done.

 

[don_resqcapt19]

Rocky,
Large cables, in my experience, have a lower meg value than small cables. As Bob said there are a lot of problems that the megger won't catch. I have seen a couple of cases where the wire was stripped down to the copper by poor installation practices and this did not show up when they were megged. It did show up under use when the conduit got water in it. Air is a good insulator and bare copper that is not touching another conductor will read good with any megger test.

The water is somebody's pipe dream...seen wet cables bounce around 3 to 5 megaohms.

That would be with the insulation intact.
Don

 

[ramsy]

megger.com has an article on Salvaging Flood Damaged Electrical Equipment. I'd wan't to read this before flooding conduits.

A nonflammable, highly conductive, and stable substitue during electrical discharge would be ionized Helium or Helium plasma, not Helium gas, which electrically insulates before properly ionized.

Plasma torches are common, but if a similar ionized Helium source is not practical, maybe Megger's product for Fault Finding Solutions for Electric Power Cables CD-ROM, offers some evidence for a more effective alternative to flooding conduits.

Megger procedures in NFPA 70B were also referenced in another puclic forum.

 

[don_resqcapt19]

Roger,

I'd wan't to read this before flooding conduits.

It shouldn't be a problem...most underground conduits are flooded under normal conditions.
Don

 

[iwire]

A nonflammable, highly conductive, and stable substitue during electrical discharge would be ionized Helium or Helium plasma, not Helium gas, which electrically insulates before properly ionized.

Plasma torches are common, but if a similar ionized Helium source is not practical, maybe Megger's product for Fault Finding Solutions for Electric Power Cables CD-ROM, offers some evidence for a more effective alternative to flooding conduits.

Yeah that sounds like a quick and cheap process.

 

[ramsy]

Yeah that sounds like a quick and cheap process.

Some of us work with people who demand more than quick and cheap. Like your domestic partner; better give em what they wan't or somebody else will.

 

[Rockyd]

Is there a written standard accept procedure for megging wire ? And what is acceptable reading for THHN?
I have a owner rep that is pissed off at my electrician on the job. He is wanting every wire megged. Which is not a problem, but he want accept our procedure. He is wanting to fill the conduits with water and meg the each wire for 2 minutes. Thank you

Don,


There seems to be more going on than meets the eye here.

1) Writtten standard for Meggering? Should have been part of the contract if this deemed reasonable and prudent (time needs to be alotted in the bid). If neccesary to turn in a submittal, I'd contact the manufacture to receive faxxed written procedures. Option two would be to see if NEIS has a written standard. IEEE or American petroleum Institue may have specifications too.

"I have a owner rep that is pissed off at my electrician on the job. He is wanting every wire megged"

What brought this on?

2)Have to be pretty rough with the wire to bring this kind of wrath I would think. I've never heard of intentionally filling the pipe (THHN is for dry and damp locations. Unless dual rated as THHN/THWN , but it's still only commercial wire in comparison to other selections available). Yes I'm fully aware that all underground fills up, especially being from Alaska originally.

3) I fail to see where two minutes is going to do a whole lot. This is not a high-pot cook out at 60K for two hours. Meggering at 1000V, on 600v conductors, for one minute is going to load up, then possibly cook out to 2 gigaohms on a low humidity day. Rain, for or high humidity days, make for poor readings of less than 1 gigaohm, 50 mega-ohms is the least acceptable reading (documentation trail) that the client is willing to accept.

I'd hate to tell you how we used to clear low meg readings (2 - 5 megs) back in the 70's onboard military ship (s) and we had an FP&E switchboard!

I have a lot of oil patch time in Alaska, and we megger everything on Alyeska contracts for pipeline work. most spaces are classified area(s). The standard call for wire is XHHW-2, for conductors in pipe, and sometimes in cabletray - flat or tri-pled up by hand. Last few years the industry has moved to HLMC and cable tray for efficiency, and cost savings.

 

[don_resqcapt19]

Rocky,
I know that a lot of specs call for megging, but I have little to no faith that a good result means that the cable is ok. If you place the leads of your megger spaced apart the thickness of standard insulation and meg at 1000 volts, you will always see an infinite result. This is exactly what can happen in a dry raceway with a small areas of bare conductor. This conductor will work fine until something changes that makes the space around the bare wire conductive. The most common thing that will do this is water. I expect that the owner in original case has some reason to suspect insulation damage.

THHN is for dry and damp locations. Unless dual rated as THHN/THWN , but it's still only commercial wire in comparison to other selections available

As far as THHN, I have never seen it where is was not dual rated as THWN...it may exist, but I have never seen it.
Don

 

[ramsy]

..The most common thing that will do this is water. I expect that the owner in original case has some reason to suspect insulation damage.

I don't believe anyone is permited to test-flood conduits without drains? NEC 225.22, 230.53, 501.5(F), 680.11
Megger-test flooding may not be permited if the water accumulation can eventually spill into junctions, panels, svc.equip, or RMC: NEC 312.2(A), 230.54(G), 352.10(D)

If drains exist, maybe metal raceways can dry before corrosion damage. Without a drain available, if test water isn't removed, there is potential for corrosion, flooding catastrophy, or electrocution.

If mother nature floods a code-compliant install, she is liable, until evidence finds some compliance issues. It seems to me, as whole food chains are sued, any discovery of conduit-flood tests --missing drains-- could expose some liability.

 

[ramsy]

Re: Helium gas.

Yeah that sounds like a quick and cheap process.

Bob, looking up at these neon and fluorescent lights I see conductive gases. Simple Helium should be cheep (fills baloons) and non toxic.

If there were no conduit drains, I would make this sell, arguing the compressed-cylender rental & ionizer fills conduits with a plazma much more conductive than water. And, gases are easily purged by a shop vac, without the compliance issues of flooding equipment with standing water.

 

[don_resqcapt19]

Roger,
My only point is that lacking a conductive enviroment within the raceway a good megger test doesn't really prove anything.
Don

 

[iwire]

Re: Helium gas. Bob, looking up at these neon and fluorescent lights I see conductive gases. Simple Helium should be cheep (fills baloons) and non toxic.

Have you found any evidence of this being done on conductors under 600 volts?

Have you priced Helium?

I can tell you it ain't like buying Nitrogen which is commonly used for many purposes in construction.

Just how would it be contained in the conduit? (Of course I am wondering the same thing about the water flood) I have to imagine the conduits that the OP was taking about where underground.

Good luck with your sales.

 

[George Stolz]

Just how would it be contained in the conduit? (Of course I am wondering the same thing about the water flood)...

I was wondering the same thing. Filling a building's raceways (with regular fittings) with water would be fun to watch. Disasterous, but fun to watch.

 

[iwire]

Filling a building's raceways (with regular fittings) with water would be fun to watch.

Seen it more than once.....it is very cool to watch.....if your not the cause or the one paying for clean up.

 

[Rockyd]

Don,

Maybe our friend, Sokkerdude, could just go with a Fluke 87 and fly through on a ohm scale reading (87's go past 40 megaohms) on a walk around, with the owners rep, to instill some confidence into him that things are okay?

Maybe in Alaska you get used to having the worst shipped out to you. I've seen some things that just make a person go "WOW". Spliced large conductors, with outter coating being the biggest part of the mechanical strength holding it together. I've seen damage by poor craftsmenship on remote sites (one wonders, and has to be scared for the wire in hiring a bunch of locals when your in BFE, or at least be able to see it from where your at).

Best wild one ever for insulation damage was at the Alaska pipeline terminal - all the pipe (GRC) had a razor sharp edge (ran length-wise) in the pipe! Pipe manufacturer paid for labor, pipe, and DAMAGED conductors that was damaged leading to the discovery of the bad pipe. Didn't need a megger to catch that one.

Call me paranoid, but I've just caught way too much trash with a megger to write them off, or pencil whip the paper. Service gear, and feeders to panel is normally it though. Real world, real construction, 15 seconds, or load up and stabilized reading, C to C's, C's to raceway) and on to the next.