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Insulation Resistance Testing and Humidity

Issue:
How does humidity affect insulation resistance testing results?

Product Line:
Circuit Breakers

Environment:
Molded case and insulated case breakers

Resolution:
High humidity can drastically reduce insulation resistance testing results for circuit breakers.

Some equipment acceptance testing documents suggest values above 100 mega ohms and normally that is okay however high humidity can drastically reduce the results of the insulation resistance values.  Below is a sample of how higher humidity can reduce the IR values, making the 100 mega ohms acceptance testing values invalid.
40% humidity:  breaker IR value above 500 mega ohms, typical 1 G ohm*
75% humidity:  breaker IR value around 10 mega ohms*
95% humidity:  breaker IR value around 4 mega ohms*

*All values above are functional and conforming. Values derived from test data on Powerpact H frame breakers in a Schneider Electric laboratory.

The IEC 60947 standard, section 6.1.3.1 states that IR testing should be done below 50% humidity and states a 1 M ohm minimum IR value for a new breaker.

NEMA AB-4 recommends IR values for breakers, less than one mega ohm (1,000,000 ohm) should be investigated.  The Schneider Electric field testing and maintenance guide also states the same one mega ohm value, as written below.

Severe environmental conditions can reduce the dielectric strength of
molded case circuit breakers. Check insulation resistance during electrical
system testing.
To check the insulation resistance, perform the following steps:
1. De-energize and isolate the circuit breaker.
2. Clean the circuit breaker as described earlier.
3. Using a megohmmeter with a capacity of 500–1000 Vdc, apply voltage
from:
a. Each phase-to-ground with the circuit breaker on (circuit breaker
contacts closed).
b. Phase-to-phase with the circuit breaker on (circuit breaker contacts
closed).
c. Between each line and load terminal with the circuit breaker off
(circuit breaker contacts open).
4. Record resistance values. Resistance values of less than one mega ohm
(1,000,000 ohm) should be investigated.
 
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