As this is a relatively low cost operation, it should be done annually, by
drawing a sample in clean bottles from near the bottom of the tank (Drain
Cock). When I was involved in this operation, as a maintenance engineer in
the 70's, the company who we allocated the refiltering and replacement of
the oil, supplied the bottles and carried out the analysis free of charge.
We drew the samples.
If the transformer is in good condition, then annual testing is
recommended. Once any increase in gases is found, then more frequent
testing is required to stay ahead of a potential failure.
We recently had a customer with a 3750 KVA transformer that tested
within limits for five consecutive years. Then all of a sudden there
was a spike that continued to rise over monthly tests. The odd thing
is that it was a problem created by Square D during the assembly of the
transformer. For five years, a very poor weld in the low voltage leads
was fine but suddenly became a hot spot. The gases helped us to
pinpoint the problem and repair it before there was a significant
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