Specifications

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consider before collecting a sample. The ideal situation for collecting a sample from an electrical
apparatus is 35°C (95°F) or higher, zero percent humidity and no wind. Cold conditions, or
conditions when relative humidity is in excess of 70 percent, should be avoided, as this will
increase moisture in the sample. Collecting a sample during windy conditions is also not
recommended because dust and debris enter the clean sample easily and disrupt accurate
particle counts. If sampling the oils is unavoidable when the outside temperatures are at or
below 0°C (32°F), it should not be tested for water content or any properties that are affected by
water such as dielectric breakdown voltage. Fluids with specific gravity greater then 1.0, such
as askarels, should be sampled from the top because free water will float. For fluids with a
specific gravity less than 1.0, such as mineral-based transformer oils, synthetic fluids and silicone
oils, the sample should be taken from the bottom since water will tend to drop to the bottom in
these fluids.
16.9 The breakdown value (BDV) for the oil :
Good oil for filling in of transformers should withstand at least 40 kV (in the above condition(vi-
a)) for one minute. If it is quite dry, the BDV could be as high as 50 kV or even 60 kV. When oil is
filled into a transformer tank and allowed to rest for several days, it will often be found that test
conducted on a sample subsequently drawn from the tank will give a much lower BDV than
before. This is because the dry oil which is filled in absorbs any moisture which may be present
inside the tank or the windings. If the BDV remains very nearly the same as before, it proves that
the windings are quite dry. As a general thumb rule, the minimum BDV for energizing any
transformer rated 33 kV or below is 30 kV. For higher voltages the minimum BDV is 40 kV. If it is
lower, it is necessary to dry out the transformer by one of the methods described earlier, until
BDV of 40 kV or more is obtained. For switch oil in circuit breakers or motor starters, etc. a
lower BDV is permissible but a minimum of 30 kV should be maintained.
16.10 Action to be taken if a transformer fails :
The action to be taken depends upon the size of the transformer, which protective relays
have operated, whether tripping is accompanied by loud noise, smoke or expulsion of oil from
the transformer, etc. Therefore, the first thing to do is to make a very careful note of the full
circumstances of the failure. The following brief notes may be of help:
i) Go round the transformer and observe its external condition, look for any damage to
the bushings, leads or cable box and any evidence of squirting of oil. Note the temperature
of oil, at the earliest, and check if the level of oil in the conservator is right. Take megger
readings between primary and secondary and also of each to earth. If everything is
right, proceed as noted below.
ii) The failure may possibly be due to sudden and heavy over-load or short-circuit. If a
HRC fuse provided for a small transformer has blown or a switch fuse has dropped
out, check if its ampere rating is right. If incorrect, replace by the correct size and
energise the transformer, after switching off the secondary circuit. If everything is all
right, close the secondary circuit ; if the fuse blows again, the fault is obviously in the