Page 395 - Industrial Power Engineering and Applications Handbook

Switchgear and controlgear assemblies 13/369
and SMC (Sheet Moulding Compounds) for the busbar The chemical thickening enables the material to be handled
mounting supports. These compounds are suitable for after the polyethylene film has been removed before
compression, transfer mouldings and injection mouldings. moulding.
They are basically fibre- or glass-reinforced thermosetting SMC is used where its superior strength and impact
plastics (FRP or GRP) and possess good physical and resistance over DMC are more important. The improved
thermal stability, high mechanical strength and excellent properties, particularly its strength, over DMC is a result
electrical properties as shown in Table 13.14. of reduced degradation of the glass and the ability to use
longer fibre. In DMC, this is usually 6-25 mm, while in
Moulding compounds SMC it is about 25-50 mm.
With the advent of these compounds in the 1960s, the The compounds so formed have excellent thermal
hitherto more conventional insulating materials, such as stability and are self-extinguishing and even completely
phenol formaldehyde (popularly known as Bakelite) and fire-retardant. Their properties are given in Table 13.14.
wood (veneered impregnated) have been almost replaced A few common types of insulators and supports are shown
by them. These compounds offer better electromechanical in Figure 13.31.
properties than conventional materials. Below we describe
the basic mix and properties of these two basic compounds, (iv) Making busbar connections
for a brief reference.
Aluminum, being a highly oxidizing and malleable metal,
requires utmost precautions when making a connection
DMC (Dough Moulding Compound) or a joint. The joint may be a fish joint for connecting
This is also known as Bulk Moulding Compound (BMC). two straight sections of a bus, tap-offs or even the
It is blended through a mix of unsaturated polyester resin, thimbling of cables on the aluminium extended links. To
crosslinking monomer, catalyst, mineral fillers and short- avoid a rapid formation of non-conducting oxide film on
length fibrous reinforcement materials such as chopped the surface of the metal, the surface must be treated
glass fibre, usually in lengths of 6-25 mm. They are all properly before making the joint. To avoid this one may
mixed in different proportions to obtain the required take the following precautions:
electromechanical properties. The mix is processed and
cured for a specific time, under a prescribed pressure 1 Clean the surface with a wire brush to loosen the
and temperature, to obtain the DMC.
oxide film and then wipe it off with a soft cloth. The
use of a wire brush serves a dual purpose; first, scraping
SMC (Sheet Moulding Compound) and removing the oxide film, and secondly, providing
This is a material produced from the impregnation of the surface with a moderate knurling (roughness),
glass fibre-mat (fibreglass, which is in the form of dry which helps to make a better surface-to-surface contact
sheet, is commonly known as chopped stranded mat and, in turn, a better joint.
(CSM)) or rovings, with a liquid and unsaturated polyester 2 Apply a contact grease with the following properties:
resin, which thickens chemically to a dry sheet form. 0 To be chemically neutral
The total mix is sandwiched between polyethylene films To have a nil or negligible electrical resistance
and then roller-pressed to impregnate and consolidate it. and

Table 13.14 Properties of thermosetting plastics
Properties Unit DMC SMC Relevant standards
for test methods

Physical properties
1 Specific gravity gdcc 1.85-1.95 1.8
2 Shrinkage mdmm 002-.003 ,0015
% 0.2-0.3 0.15
3 Water absorption % 0.2 (m.) 0.15 (m) BS:2782
4 Operating temperature "C 140-150 140-150
Mechanical properties
1 Minimum tensile strength kgf/cm2 250-500 50&900 BS:2782
2 Minimum compressive strength kgflcm2 1200-1800 1600-2000 BS:2782
3 Minimum cross-breaking strength or kgflcm2 700-1200 1400-1 800 BS:2782
flexural strength (bending strength)
4 Impact strength kgf cdcm2 30-40 60 BS:2782
Electrical properties
1 Dielectric strength (min.) kVlmm 10-14 10-14 BS:2782
2 Tracking index V 1000 1000 BS:5901
Note These values are only indicative and may vary with the quality of mix and process of curing etc., and differ from one manufacturer
to another. For exact values, contact the manufacturer.