Need help with new megger

Status
Not open for further replies.

Ed Carr

Senior Member
Location
way upstate NY
Having read a number of posts concerning the importance of a megger
I bought a new Fluke 1520 off ebay.Now I know a number of people
here a quite well versed on the use of such equipment.I however am
green as it gets.The factory guide has very little info in it.Does anyone
have one of these meters or something similar? Is there anyone here who
would be interested in giving me a little crash course and or pointers on
the proper use and dangers associated with this? Thanks,Ed
 

Ed Carr

Senior Member
Location
way upstate NY
Thanks 480. I have already looked at this from some previous posts
on the subject.Ive read through a good part of it again tonight.
Hopefully someone could pm me that would be interested in talking
me through some of the test procedures.
 

frizbeedog

Senior Member
Location
Oregon
480sparky said:


Megger Marc! Paging Megger Marc!​
Suggestion for a sticky: All questions concerning meggers should be directed to Marc via PM.

We love ya Marc, but I suggest we have a new forum category called Meggering. Moderated by Marc. aka Megger Marc.

Given the intrest lately, it should keep you busy.

If they don't tease you they don't really like you.

:D :wink: :roll: :cool:
 

zog

Senior Member
Location
Charlotte, NC
Ed Carr said:
Having read a number of posts concerning the importance of a megger
I bought a new Fluke 1520 off ebay.Now I know a number of people
here a quite well versed on the use of such equipment.I however am
green as it gets.The factory guide has very little info in it.Does anyone
have one of these meters or something similar? Is there anyone here who
would be interested in giving me a little crash course and or pointers on
the proper use and dangers associated with this? Thanks,Ed
What are you planning on testing?

Keep in mind you are using a DC voltage, so there are 3 different compoents (Actually they are currents that you are measuring, there is no such things as an ohmeter) that you will be measuring that will dissipate over different times (Besides leakage current that is steady) so you need to understand what you are reading. Depending on what type and voltage of equipment you are testing you may need to sperate these components and calculate DAR and PI ratios to get a true evaluation of the insulating system.

I must also stress the importance of isolating what you are testing from other equipment, not only can you damage other equipment, your readings will be incorrect.

And the #1 most important thing, temperature correction!! Any spec you look at from any source is assuming your readings are temp corrected to some value (Usually 20 degrees C).
 
Zog
When measuring the values for conductors (primarily THHN/THWN), that are generally 50ft-300ft, how important is the temperature factor you have mentioned?
I generally work in temperature ranges of 40F-100f through the year. Also, the humidity level is from [40F-30%] to [100F-90%]. I know that is a huge swing. Is there a chart for something like this, or does it require some kind of calculation?
 

Ed Carr

Senior Member
Location
way upstate NY
Zog,I bought the meter to do some work on parking lot lights.
Due to a lot of digging at the site i've been on at least six service
calls.We now have a breaker that won't reset.I believe my Fluke
is not a true megger but an insulation tester.It has two leads only
with no guard terminal.Anyways to answer your question I want
to start with testing the integrity of wire insulation.Thanks for your
interest and help!
 

480sparky

Senior Member
Location
Iowegia
frizbeedog said:
Suggestion for a sticky: All questions concerning meggers should be directed to Marc via PM.

We love ya Marc, but I suggest we have a new forum category called Meggering. Moderated by Marc. aka Megger Marc.

Given the intrest lately, it should keep you busy.

If they don't tease you they don't really like you.

:D :wink: :roll: :cool:
Better yet, maybe Marc should start having classes and make some bucks.....:D
 

zog

Senior Member
Location
Charlotte, NC
Pierre C Belarge said:
Zog
When measuring the values for conductors (primarily THHN/THWN), that are generally 50ft-300ft, how important is the temperature factor you have mentioned?
I generally work in temperature ranges of 40F-100f through the year. Also, the humidity level is from [40F-30%] to [100F-90%]. I know that is a huge swing. Is there a chart for something like this, or does it require some kind of calculation?
Here is the formula for solid insulation:

Rc = Ra x K
Where: Rc is resistance corrected to 20? C
Ra is measured resistance at test
temperature
K is applicable multiplier

Here are the Mulitpliers:

Test Temperatures to 20? C
Temperature Multiplier
? C ? F Apparatus Containing Immersed Oil Insulation Apparatus Containing Solid Insulation
-10 14 0.125 0.25
-5 23 0.180 0.32
0 32 0.25 0.40
5 41 0.36 0.50
10 50 0.50 0.63
15 59 0.75 0.81
20 68 1.00 1.00
25 77 1.40 1.25
30 86 1.98 1.58
35 95 2.80 2.00
40 104 3.95 2.50
45 113 5.60 3.15
50 122 7.85 3.98
55 131 11.20 5.00
60 140 15.85 6.30
65 149 22.40 7.90
70 158 31.75 10.00
75 167 44.70 12.60
80 176 63.50 15.80
85 185 89.789 20.00
90 194 127.00 25.20
95 203 180.00 31.60
100 212 254.00 40.00
105 221 359.15 50.40
110 230 509.00 63.20


Lets say you meggered a cable and the result was 140M, and your spec says must be >100M. At 41 degrees F the actuall insulation resistance value is only 70M but at 95 degrees F it is 280M, so yeah it makes a big difference.

Humidity also affects the readings but there is no correction formula for that, so we always try to test under similar conditions, if the tesing was last done 3 years ago in the winter (Dry) we recommend to do the testing around the same time of year, most plants do this anyways because they have thier scheduled shutdowns.

Edit: Sorry that table is so hard to read, try the attachment.
 
Last edited:

zog

Senior Member
Location
Charlotte, NC
Ed Carr said:
Zog,I bought the meter to do some work on parking lot lights.
Due to a lot of digging at the site i've been on at least six service
calls.We now have a breaker that won't reset.I believe my Fluke
is not a true megger but an insulation tester.It has two leads only
with no guard terminal.Anyways to answer your question I want
to start with testing the integrity of wire insulation.Thanks for your
interest and help!
Megger is a brand name that everone uses for a megohmeter, your Fluke 1520 is just fine for LV testing. LV cable testing is pretty basic, to use Brians term from another post "Divide and conquer", test for 60 seconds or until the reading is stable, dont Megger your boss, temp correct, and know exactly what you are testing before you hit the button, electronics dont like being meggered.

Edit: What kind of breaker are you having problems with? Might be an easy fix.
 

mdshunk

Senior Member
Location
Right here.
A more specific question, like what is it exactly you want to do/test, might yeild a better answer. Generally, everything there is to say has been written and rewritten in the past year or so here.
 

Ed Carr

Senior Member
Location
way upstate NY
OK say I want to check the wires to the first pole base from the panel.
The lights are @ 277volts.I have three #4thhn wires in pvc conduit with
I'm sure a fair amount of moisture.Am I correct that the procedure is to
connect one lead to the bare conductor and the other lead to the insulation?
I should disconnect all wires at each end?What voltage would I apply and
for how long.I told you I was green at this.Hope I don't come across as too much of an idiot!
 

mdshunk

Senior Member
Location
Right here.
Ed Carr said:
.Am I correct that the procedure is to
connect one lead to the bare conductor and the other lead to the insulation?
No, you never connect to the insulation, unless it's something like the screen on HV cable.

In your case, with your 277 lights in PVC conduit, there's only a few checks you can make. First, check from each conductor to the EGC:
L1 to EGC
L2 to EGC
L3 to EGC
N to EGC

You can also check phase to phase, since you don't have any 480 loads:
L1 to L2
L2 to L3
L3 to L1

Do not check phase to neutral, since your ballasts are still hooked up. You have a pretty good chance of smoking something. If you care to disconnect all the conductors from each other at each pole base, you can check phase to neutral if you want to. I'd do these checks at 1000 volts, and I'd want to see greater than 10 megohms.


EDIT.... It might go without saying, but you do these checks with the power off, and the conductors disconnected at the feed end. Megger from the feed end. If you find an 'issue', take something apart midway in the run and megger each direction to start to narrow down the exact section with the issue.
 

Ed Carr

Senior Member
Location
way upstate NY
Marc,Thank you! Now to clarify.I have 3 wires leaving the panel.
One hot,one neutral and the ground. I can test between the hot and
ground or the neutral and ground correct? To test between the hot
and neutral I have to disconnect at both ends. I figured I could smoke
the ballasts with them still in the circuit.Again Marc how long do I hold
the test button for? I've had more detailed instructions come with
a toaster. I just want to make sure I do this right. Thanks, Ed
 
Status
Not open for further replies.
Top