When & what to megger

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rancho5000

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Guys...I am looking for some opinions on when and what to megger. I build small concrete shelter with electrical in them. One in question is a 12'wide x 36'long, the longest run is maybe 30'. I ship these shelters all over the U.S. This one is going to Texas and the inspector is requesting us to megger these short branch circuits. Everything is ran in emt, surface mounted. We have never needed to megger branch circuits before but this time he is making us. Personally I don't think it is necessary on these short runs. Plus everything is already terminated and has been energized for hot check. Now we are having to de-term the entire shelter.

Is there a rule of thumb as to what a length of wire run to megger on? For instance, say anything over 100' should be megger tested. Is it really necessary to megger something this small?

Just curious on feedback from experienced in this field.
 
Seems like he requests it because manufactured structures (such as, for example mobile homes that contain electrical) are megged If I'm not mistaken as a proven way to record the condition of the wiring before it's taken possession of.

Could it be a requirement for anything manufactured?
 
When & what to megger

You asked for Opinions so here is mine. This isn’t based on some code or State requirement, just on my experience with Customer requirements over the years and personal practices.

From my experience, I occasionally get requests to Meg and Record on Outdoor and Portable Systems. The requests are usually from older generation Customers that were around when THHN wasn’t available or thought of. I have had requests to Meg individual or individualized Circuits and Record the results to prove that there are no shorts to ground at a given Voltage; usually the 1,000 Volt Scale. This is a customer requirement only, you should charge for it as you will need to File and Store results for future references even after Warranty Periods.

When I run Three Phase, I always Meg before a Power up test. Watch the Listed Voltage capabilities on Receptacles, Switches, etc and don’t exceed that Rating. I wouldn’t Meg with LED or other Light Sources installed; just the bare circuits as the Megger can damage equipment.

If you are running 208 or 240 Circuits I would Meg at the 500 Volt Scale. With 480 I would Meg at the 1,000 Volt Scale; in other words, Meg with twice the voltage that will be applied to the Circuit. Look at Circuits that has any Megger Reading that isn’t close or near Infinity. Something is breaking down or there is some piece of equipment connected.

If you are Megging and the needle Bounces low (half Meter Scale or less) then Bounces back to a good reading. Something in the Circuit is Jumping / Arcing to Ground (Meter Reading goes down) that temporally lowers the Megger Voltage and the Meter Swings back towards Infinity. Continued Cranking on the Megger repeats the Cycle from Good to Bad.

I would shop for the old fashioned Hand Crank Meggers. There are good used ones on the eBay Market and new Knockoff / past Patent Date Chinese models on eBay. Look for an Analog Meter and Selectable Scales that fit into what you are wiring. I would also keep the Megger in its Case and in the Cab of the Truck.

JimO
 
When & what to megger

That's a good question. No, the Device's real name is Megohmmeter or Meg Ohm Meter and is shortened to Megger. A good Brand is Biddle that has been around for a very long time. The Chinese model I spoke of is listed as, "New Megger, Insulation Tester 1000 V meter. Sells for under $75 on eBay.
The 75 Year old Models have a black Bakelite case. The Meter has a hinged metal cover to prevent damage and Folding Handles to allow full view of the Meter when in use.

I prefer an Analog Meter as the Digital has a tendency to try and show you the fluctuations in the reading and comes up all '8' most of the time. I prefer the hand crank model, it never needs Batteries.

JimO
 
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