Megger testing THHN in underground RGS

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Fordean

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Can someone direct me to site explaining proper procedure for this. I would like to verify. Do you clamp to Rubber insulation in some cases. ?
 
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petersonra

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Fordean said:
Can someone direct me to site explaining proper procedure for this. I would like to verify. Do you clamp to Rubber insulation in some cases. ?
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Is THHN rubber? I thought it was thermoplastic.

In any case, THHN is not allowed underground.

THWN is, and most THHN is dual rated as THWN, but if it is really THHN you have a bigger problem than figuring out how to to do an insulation test.
 
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Fordean

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New Jersey
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Electrical Contractor
petersonra said:
Is THHN rubber? I thought it was thermoplastic.

In any case, THHN is not allowed underground.

THWN is, and most THHN is dual rated as THWN, but if it is really THHN you have a bigger problem than figuring out how to to do an insulation test.
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Is dual rated thwn/thhn Pretty sure
 
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Fordean

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brian john said:
I have no idea what you are asking or trying to say?
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In examples I have seen doesnt say type of cable they are testing in examples.. THey use the wore "LEAD" when clipping to the Outer jacket of cables. But dont specify what the outer cable is made of by not disclosing cable type they testing. Lead cant be looked at two definitions
 
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Strathead

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Fordean said:
In examples I have seen doesnt say type of cable they are testing in examples.. THey use the wore "LEAD" when clipping to the Outer jacket of cables. But dont specify what the outer cable is made of by not disclosing cable type they testing. Lead cant be looked at two definitions
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If you understand electrical theory, perhaps this will help. The purpose of "meggering" is to determine the integrity of the insulation around wire. To do this you merely conduct a resistance test to determine the resistance of the insulation. Like any resistance test, the meter puts a voltage from one lead to the other and reads the current flow. Using ohms law, the meter reads the result of E/I= the R you are looking for. When meggering, you are hoping for the highest results possible. Since you are testing for insulation resistance, you want to use a voltage that exceeds the voltage that you will be using in normal insulation.

So, you want to hook the meter leads up to any an all places where the voltage could "leap" across the insulation. If you have a shielded lead, or a shielded cable, the you should check from lead to shield. If you have lead together in a cable or a conduit then you want to also check from each conductor to the other conductors. If you have metal conduit, you want to check from each conductor to the conduit. It is always better to check in the morning, when the air is moist, since, if there is a failure, you want it to be during testing, not later when operating. Many specifications that I have seen including Nuclear Submarine wiring is an insulation resistance of 100,000 ohms or more. I have rarely seen an insulation resistance less than 1,000,000 ohms.

Does this help answer your questions?
 
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Fordean

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Location
New Jersey
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Electrical Contractor
Strathead said:
If you understand electrical theory, perhaps this will help. The purpose of "meggering" is to determine the integrity of the insulation around wire. To do this you merely conduct a resistance test to determine the resistance of the insulation. Like any resistance test, the meter puts a voltage from one lead to the other and reads the current flow. Using ohms law, the meter reads the result of E/I= the R you are looking for. When meggering, you are hoping for the highest results possible. Since you are testing for insulation resistance, you want to use a voltage that exceeds the voltage that you will be using in normal insulation.

So, you want to hook the meter leads up to any an all places where the voltage could "leap" across the insulation. If you have a shielded lead, or a shielded cable, the you should check from lead to shield. If you have lead together in a cable or a conduit then you want to also check from each conductor to the other conductors. If you have metal conduit, you want to check from each conductor to the conduit. It is always better to check in the morning, when the air is moist, since, if there is a failure, you want it to be during testing, not later when operating. Many specifications that I have seen including Nuclear Submarine wiring is an insulation resistance of 100,000 ohms or more. I have rarely seen an insulation resistance less than 1,000,000 ohms.

Does this help answer your questions?
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Yes thanks helped alot. Better then most tutorials I am viewing. Next question is. Isolation of test. Can I megger up to a Motor Starter Without Disconnecting. I know You can go through anything like VFD's, Caps, Or transistor based circuitry. I want to avoid Disconnections. If possible.

Also would like to Discribe why I am doing this. In next post topic I will make. Anything additional comments appreciated.
 
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Strathead

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Location
Ocala, Florida, USA
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Electrician/Estimator/Project Manager/Superintendent
Fordean said:
Yes thanks helped alot. Better then most tutorials I am viewing. Next question is. Isolation of test. Can I megger up to a Motor Starter Without Disconnecting. I know You can go through anything like VFD's, Caps, Or transistor based circuitry. I want to avoid Disconnections. If possible.

Also would like to Discribe why I am doing this. In next post topic I will make. Anything additional comments appreciated.
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I think you had a typo. Don't go "through" VFD's etc. They should be disconnected. If the motor starter or VFD has a separate control circuit than the feeders you are meggering, then fine, but otherwise disconnect.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
Fordean said:
Yes thanks helped alot. Better then most tutorials I am viewing. Next question is. Isolation of test. Can I megger up to a Motor Starter Without Disconnecting. I know You can go through anything like VFD's, Caps, Or transistor based circuitry. I want to avoid Disconnections. If possible. ...
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You cannot megger conductor to conductor without isolating the conductors. Typically no disconnection required for a disengaged motor contactor, but disconnecting at the motor end is inevitable.
 
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