TO TEST OR NOT TO TEST; WATCH FOR MOISTURE
In wet environments, industry associations are conservative about dc testing.
- Published in Southwire's Power Cable Update newsletter in October 1997
- Reprint permission granted
High-potential dc testing of cross-linked polyethylene (XLPE) medium-voltage cable is a good installation check. But in wet environments you may want to steer clear of hi-pot testing once the cable is in service.
"Three major industry bodies agree on hi-pot testing as an installation test," says Sid Ticker, senior applications engineer. The Institute of Electrical and Electronic Engineers (IEEE), the Association of Edison Illuminating Companies (AEIC), and the Insulated Cable Engineers Association (ICEA) all have standards that address dc field testing of medium voltage (MV) cable.
After installation: reduce test stresses
"The three groups are in close agreement about test values for installation tests," Ticker says. "After installation, though, all three reduce testing stress and add cautions about wet environments." (See table.)
IEEE 400 describes maintenance testing of up to 15 minutes, but it also points out that dc testing may be detrimental to cables that have been subjected to long periods of exposure to moisture.
AEIC CS5-94 describes an after-installation test for the first five years, but adds the statement, "After that time, dc testing is not recommended."
ICEA S-66-524 says, "If voltage tests are made after installation, they shall be made immediately." S-66-524 makes no mention of maintenance testing. A new draft ICEA standard, ICEA S-94-649-focused on concentric neutral designs-does specify maintenance testing limited to five minutes.
A project report by the Electrical Power Research Institute (EPRI report TR-101245) recommends against dc testing of XLPE-insulated cables where water-treeing may exist. The EPRI report was limited to XLPE cables aged under wet conditions.
Industry practices vary
Maintenance testing practices vary among users of medium-voltage cable. Industrial users frequently use hi-pot testing to check aging cable. There is at least one study that shows that this practice can contribute to overall system reliability.
"There is no evidence that dc tests are harmful to NW XLPE insulation that has been operating in a dry environment, even after five years," Ticker says. On the other hand, utility users are moving away from the practice of hi-pot cable maintenance testing.
What's the difference? Moisture in the application environment. Industrial applications tend to be dry environments where water-treeing isn't a problem. Utility applications lean toward direct burial of XLPE-insulated cable. The wet underground environment generally favors the formation of water trees in the cable insulation. These are the applications where post-installation testing is questioned.
"The exception to that generality is water-impervious cable designs," Ticker says. "Where a water-impervious metallic sheath or covering keeps all moisture away from the cable insulation and insulation shield, water treeing isn't a problem." Hi-pot maintenance testing doesn't seem to cause damage.
DC Field Testing XLPE 15kV 133% Insulation |
Standard |
During Installation |
After Installation |
IEE 400 |
56kV/15 minutes |
46kV/5-15 minutes |
AEIC CS5-94 |
64kV/5 minutes |
20kV/5 minutes1 |
ICEA S-66-524 |
65kV/15 minutes |
none specified |
ICEA S-94-6492 |
64kV/15 minutes |
20kV/5 minutes |
1 First five years of operation |
2 Currently submitted to ANSI for approval |
|
