HOW MANY MEGOHMS DO YOU NEED?
Megohmeter readings of 600V insulation need interpretation.
- Published in Southwire's Power Cable Update newsletter in January 1998
- Reprint permission granted
Good 600V insulation has high resistance. Insulation in poor condition has lower resistance. But how low is low? It depends.
"A 'good' insulation resistance reading is always an interpretation," says Dave Mercier, Southwire applications engineering manager.
What's the uncertainty? To begin with, different wire types show wide variations in insulation resistance. Also, different insulation types respond to temperature differently. The table examples illustrate the variations.
Your Actual Resistance May Vary
If you allow for insulation type and temperatures, can you compare your measurements with resistance tests made in the factory? Not necessarily. Factory tests have optimum conditions. In the field, moisture, contamination and cable taps can lower resistance readings significantly.
"Field conditions may cause megohmmeter readings that vary widely from factory test numbers," says Mercier. "You should look for trends over time rather than absolute values. It's valuable to make an installation check, then use that as a reference point for periodic checks."
If you want to make standardized comparisons, you can calculate the insulation resistance (IR) for 1,000 feet of conductor with this formula:
Where L = conductor length in feet;
R = megohmmeter reading, in megohms; and
f = temperature correction factor. (See table.)
Overall readings can be indicators
"A good cable installation should give you overall insulation readings of at least 50 megohms," says Mercier. "If you see less than that, you may not have a problem, but you should do some checking to find out where the
leakage is occurring."
What do you look for? If possible, check the conductor for damaged insulation. Conductor ends that are dirty or damp may need to be cleaned and dried.
"Care in field resistance measurement pays off," says Mercier. "Properly interpreted, it's a useful field test."
Temperature Correction (f) |
Type XHHW, RHH, RHW, USE |
TEMP (°F) |
Type THHN |
|
55 |
0.86 |
0.75 |
60 |
1.00 |
1.00 |
65 |
1.17 |
1.34 |
|
