Page 387 - Industrial Power Engineering and Applications Handbook

131362 Industrial Power Engineering and Applications Handbook
conducting a short-circuit test up to the desired duration Table 13.12 Derating for higher altitudes for metal enclosed
of 1 or 3 seconds, the short-circuit current, although switchgear assemblies and bus systems
theoretically asymmetrical until the test period, may be
regarded as symmetrical (having reached its steady state) Altitude (mi ~ Derutitig factors
after three or four cycles. The content of asymmetry is LT systems HT systems 1 Dielectric strength Continuous current
ignored after three or four cycles for all calculations and rutirig
practical purposes. In fact, a d.c. component less than
50% that of the peak symmetrical component of the fault, 2000 1000 1 .0 1 .OO
current Is,, at any instant during the course of short- 2600 1500 ~ 0.95 0.99
circuit condition may be ignored. In other words, the 3900 3000 0.80 0.96
relevance of the asymmetry may be considered only up Notes I Intermediate values may be obtained by interpolation.
to the first peak, as the immediate subsequent peaks may 2 No derating i, applicable in case of insulated switchgear
also be less than 50% of the peak value of Is, (Idc < 0.5 (GIS) assemblies.
ZM) at that instant. Source ANSI C-37.20.2 and C-37.23
The generation of an asymmetrical current on an a.c.
system, leads to the inference that a short-circuit condition increasing the clearances and the creepage distances to
will give rise to a d.c. component due to a shift in its ground and between phases as discussed in Section 28.5.2
zero axis. During the sub-transient state the value of the and Tables 28.4 and 28.5.
asymmetrical current will be the phasor sum of the To achieve the original value of the continuous current,
symmetrical Is, and the asymmetrical Idc, current the size of the current-carrying components may also be
components. For details refer to Section 14.3.6. increased to carry the higher amount of current in
proportion to the derating.
13.4.2 Service conditions
Atmospheric conditions
Ambient temperature, altitude and atmospheric conditions A normal enclosure is meant for a reasonably clean
at the place of installation of electrical equipment are atmosphere and a relative humidity not more than SO%
considered to be the service conditions for the equipment for LT and 95% for HT indoor enclosures. Where the
to operate and perform its duties. All electrical equipment atmosphere is laden with fumes or steam, saline or oil
is designed for specific service conditions and variations vapours, heat and humidity, excessive dust and water or
may influence its performance. Below we analyse the contaminated with explosive and fire hazardous gases,
influence of such non-standard service conditions on the vapours or volatile liquids (Section 7.11) a special
performance of equipment and the required safeguards enclosure with a higher degree of protection is required
to achieve its required performance.
as in IEC 60529 or IEC 60079-14. For non-hazardous
areas, the enclosure can be generally one of those discussed
Ambient temperature in Tables 1.10 and 1. I 1, and when required can be provided
with special treatment to the metallic surfaces. For
The rating of an indoor or outdoor switchgear assembly hazardous areas, however, special enclosures will be
is referred to at an ambient temperature of 40°C. For a essential as discussed in Section 7.11.
higher ambient temperature the rating of the assembly In outdoor type switchgear or controlgear assemblies
will be reduced in the same proportion as for the busbar the normal practice is to provide a double door in the
systems and as shown in Table 28.3.
front to house the front panel and protect the door knobs,
meters, lights, pushbuttons, reset knobs or other acces-
Altitude sories mounted on the door and thus prevenl water or
dust leaking through joints, knockouts and fitments etc.
It is also recommended to have a canopy on the top of
For assemblies using the surrounding air as the the enclosure to protect the panel from direct rain. Figures
insulation and cooling medium 13.6 and 13.28 illustrate this type of construction.
The standard altitude for an LT switchgear assembly is
2000 m and for HT 1000 m, as in ANSI-C-37.20C and Flame- or explosion-proof assemblies (Type Ex. d)
IEC 60439-1. At altitudes higher than this, the normal
increase in temperature will become greater and the For hazardous areas flameproof enclosures alone are
dielectric strength less for all assemblies using air as the recommended, except in areas with moderate intensity
insulating and cooling medium. For applications at higher of contamination and where such assemblies are located
altitudes, therefore, the derating factors as noted in Table away from the affected area and in a separate well-
13.12 must be applied to obtain the reduced level of ventilated room, when pressurized enclosures may also
dielectric strength, i.e. one-minute power frequency be safe. The reason for this precaution is that frequent
voltage withstand, impulse voltage withstand and the arcing takes place within the enclosure on each switching
continuous current rating etc. to which level the switchgear of a contactor, switch, breaker or an OCR etc. and also
assembly will now be rendered to. To achieve the original during operation of power and auxiliary contactors.
level of dielectric strength, the insulation system of the The classification of gases, vapour and volatile liquids
switchgear assembly will have to be improved by the according to their ignition temperatures has been given
same degree as the derating. This can be achieved by in Table 7.4. The basic requirements of these enclosures,