Page 396 - Industrial Power Engineering and Applications Handbook

13/370 Industrial Power Engineering and Applications Handbook

(c) Any other chemically neutral grease, which
will have no electrical resistance and which
can withstand a minimum tracking tempera-
ture of 200°C.
3 The joints must be tightened with a torque wrench.
For the recommended torque values refer to Table
29.1.

Note
Petroleum jelly is not recommended due to its low tracking
temperature. The minimum tracking temperature is recommended
to be 200°C, the same as for the busbars during a fault. Also refer
to Section 28.4.1.

Fasteners
Only high tensile (HT) fasteners must be used for busbar
jointing and their interconnections or links not only to
take care of the fault level but to also maintain the
recommended contact pressure over a long period of
Figure 13.31(a) Insulators to hold busbars in flat configuration operation as noted in Table 29.1. An ordinary fastener
(Courtesy: J.K. Plastics) may not be able to withstand or sustain this torque for
long. Similarly, the busbar supports, which are mounted
on only two or three fasteners, should also be fitted with
these fasteners.

Electroplating of HT fasteners
HT fasteners in normal manufacturing are black
phosphated and then lubricated. They are not required to
be electroplated, as they do not rust unless the phosphate
... coat itself is damaged. Such fasteners when used for
electrical purposes, such as for mounting and jointing of
busbars and their supports, are not generally exposed to
outdoor conditions. The phosphate coating thus remains
intact and an electroplating (zinc or tin passivation) is
not required.
Moreover, electroplating of HT fasteners may pose
the problem of hydrogen embrittlement, which can cause
Figure 13.31(b) Insulators to hold busbars in vertical configuration cracks on their surfaces. The HT fasteners are already
(Courtesy: Vinayak Gorp.) heat-treated and have a high content of carbon. When
they are electroplated, whatever hydrogen they may emit
during acid pickling is trapped on the surface, as it forms
a strong bonding with the carbon. The C-H bonding
renders the surface brittle. Rapid removal of hydrogen
therefore becomes essential to save the hardware from
surface cracks. It is possible to do this by tempering the
hardware at a low temperature, say, 100-120°C for about
30 minutes, before transferring them to the electroplating
bath.
If such fasteners are stove-enamelled (which is normally
not done), the trapped hydrogen is removed automatically
Figure 13.31 (c) Conical insulators (Courtesy: J.K. Plastics)
while being stoved. Since the fasteners are used only for
the assembly of switchgear or busbar systems, they are
used at room temperature only. Therefore, if they are
To have a minimum tracking temperature (at electroplated, they must be tampered, which is time
which the grease may start conducting) of 200°C. consuming and adds to the cost of production. Moreover
Apply this grease swiftly after the surface clea- this has no technical advantage. The HT hardware must
ning to avoid a fresh oxidation. The following therefore be used as they are. When it is absolutely
are a few types of greases: necessary to electroplate them, tampering will be essential.
(a) Servogem 2 (multipurpose) from the Indian
Oil Corporation 13.6.2 Service conditions
(b) Multipurpose grease H from Hindustan
Petroleum Ltd or These are the same as discussed in Section 13.4.2. For