GO/ MARGINAL/NO GO MEGGER READINGS

[Ragin Cajun]

We have an issue on parallel 500 MCM cables at a hospital. Seems the "electrician" pulled them many months early and failed to properly protect them where they exited a conduit. This is just one of numerous issues with this electrician.

We are requiring him to megger them. Voltage is 480V, so we plan to megger at 1000V for 60 seconds, phase to phase and ground, six tests per conductor.

What values do you typically use for go/marginal/no go?

The last time we had testing done typically the cables were 400meg ohms to infinity. I have a bad feeling these will test out low. But how low to require the electrician to re-pull the cables? I know he will really fight re-pulling. I expect that if we consider the readings "too low" we likely will need to bring in a specialized testing contractor. Could be nasty.

We have reviewed some of the literature and some of the "acceptable" values look way too low to what I am used to seeing.

Thanks ahead of time,


RC

 

[don_resqcapt19]

Properly protect them from what?

Even if they have bare copper showing, as long as the conduits are not filled with water or another conductive substance like some wire pulling lube and as long as the bare copper is not touching conductive parts, they will megger just fine.

 

[zog]

Per the ANSI/NETA Acceptance Testing standard

Insulation-resistance values shall be in accordance with manufacturer?s published data. In the absence of manufacturer?s published data, use Table 100.1.Values of insulation resistance less than this table or manufacturer?s recommendations shall be investigated.

Be sure to tempature correct your reasings to 20 degrees C, makes a big difference. For example, lets say it is 32F (0C) like it will be this weekend, and your test result is 100 Meg, corrected that is only 40 Meg.

 

[Sierrasparky]

Properly protect them from what?

Even if they have bare copper showing, as long as the conduits are not filled with water or another conductive substance like some wire pulling lube and as long as the bare copper is not touching conductive parts, they will megger just fine.

I would agree unless there is moisture a megger is not going to tell much. I don't see how the conductors could be damaged inside the conduit. If the exposed portions have some nicks in the insulation that can be remidied by using proper materials.




Were these conductors left dangling at the floor were the conduit emerged, or even outside? or are they just hanging out of a pull box.

 

[Ragin Cajun]

Regarding why the damage to the conductors, I haven't been to the site and after several requests for pictures - nothing.

From what little information I can gather it seems the contractor pulled the conductors through the conduit installed at that point in time and was waiting to complete the conduit run. Sounds like he left them hanging out of the conduit unprotected, no plastic bushing, etc. STUPID!!!

We will be going to the site in a couple of weeks. He doen't plan to finish the pull until late Jan or early Feb. He claims to have over $100,000 in cables on this run so he is antisy. Seems stupidity is in force there.

I don't know how he will continue from where he is now. Hopefully will find out when we get there. Should be "interesting".

RC

 

[zog]

You can try a nitrogen purge

 

[ATSman]

You can try a nitrogen purge

All cables will absorb moisture if the ends are not sealed. They do not even have to be lying in water.
We had a similar situation with MV cables on a 3 phase run between 2 manholes. We made up a manifold using the insulating boots from large alligator clips and hoses to connect the nitrogen bottle to the 3 cables. With only about 2 lbs pressure set on the regulator it worked great. It took 2 weeks of purging before we were able to get decent readings on the 15KV megger.

 

[junkhound]

As others have said, megger test is useless in this situation.

Also, what about "He doesn't plan to finish the pull until late Jan or early Feb" ??

I'd run a 1500Vac hipot test for 60 sec now and another after all cable is pulled to verify installation. With $100K in cables and hospital reliability at stake involved, git yerself over to the site and do a personal inspection if you are the 'final' responsible person.

 

[cadpoint]

Yeah why wait the clean pony tour, go see what the real conditions that are out on the job.

And as said if they ran conductors, what else are they going to hide from you and the pony tour?

The new style of office people/management is to show up on Job and walk through all hours of the day.

Your post made me ant?sy...

 

[don_resqcapt19]

What damage does water do inside the cable? As long as it doesn't run out and damage the equipment at the terminations, does it really do any harm?

I remember removing so old 5kV cable from the service equipment in a factory. They were installed back in the time when it was common for the conductor to bus termination was completely taped up. When removed the tape so we could make the disconnections, about a pint or more of water ran out of each of the cables. At the outside connection to the utility, the conductors had been terminated with the open end of the conductor pointing up using a "eye-bolt" type termination. There was a ~20' rise between the switch gear and the outside termination point and over time the space between the strands had filled up with water.