Std = 2Ktd (Slm + Sls )
Where Ktd is a marginal factor to account for future increase in load, assume Ktd to be 1.3.
Assume the leakage reactance Xtd of each transformer to be 0.055 per unit, and ignore the
18.104.22.168 Equivalent transformer
230 HANDBOOK OF ELECTRICAL ENGINEERING
transformer for this relay has a primary current rating of 150 amps. Assume only one of the
transformers is operating. Let the route length of the motor cable be 250 metres and
assume that the
volt-drop in the c
Where l is the route length of 250 metres
V is the rated line voltage of 440 volts
cosstm is the starting power factor of 0.33
and sinstm = 0.9440.
0.33r + 0.944x 440 15.0
3 1408 100
0.10825 ohm/km for the cable.
Zbase = _R2
= Xbase_0.12 + 1.02 = 1.00499Xbase
Xbase = 0.8067
= 0.8027 ohms/phase
Rbase = Xbase
= 0.08027 ohms/phase
Transfer these components to the secondary circuit at 460 volts, and call them Zbs
Zf = Zbs + Zt = 0.001054 + j0.006821 ohms/phase
The magnitude of this is 0.006902 ohms/phase, which has an X/R ratio of 6.4715. The
2 _1.0 + e
_ = 2.2846
The prospective RMS fault current at or near to the fuses is If ,
of ingress protection. The degree will depend upon whether the equipment is to be installed
and exposed to the extremes of the weather, or indoors and exposed (or not) to dust or
The degree may also depend upon whether the equipme
a prescribed pressure inside the enclosure.
The purging air must be drawn from a non-hazardous area source e.g. through suitable
or from a plant air compressor. If the enclosure is large, as in the case of high voltage
the use of plan
than the normal load current even if it is its full-load value. Hence Ifl in (9.6) can be ignored,
224 HANDBOOK OF ELECTRICAL ENGINEERING
giving the form of equation found in the reference literature,
Isc = AK
A = Isc
care so that the machined surfaces are not degraded or damaged. BS5345 was introduced in
address this and similar subjects. See Reference 4 for practical view of the problems
maintenance of hazardous area equipment.
10.5.2 Type of Pr
of fuses and moulded case circuit breakers for their let-through current as calculated from
When a fuse is used in a motor circuit its main purpose is to protect against short circuits in
the cable and not against overloading of the
Ist = 7.0 Ifl = 7.0 219 = 1533 amps
From (9.5) the route length lst is,
lst = 415 15.0
3 1533 (0.342 0.35 + 0.0754 0.9368)
= 0.1229 km = 123 metres.
It can be seen that starting current determines the route length to be no greater than 123
When gases and vapours are present in air the resulting mixture may be flammable. Not all
and vapours produce flammable mixtures. Imagine a flammable gas or vapour slowly
leaking into a
confined volume of air that is not replenished. Initially the c
cos(t + ) = 0
Now since also approaches zero cos t equals zero for the first time when t = /2.
If the above conditions are substituted into (11.5) the current becomes,
sin (t + (0 0) =
Which is in phase with V as can be expected.
E) and appropriate reactance X (X
or Xsd ) are
known. Hence the symmetrical fault current If may be easily calculated:
If = E
A 6600 V, 4.13 MVA generator has X
= 15.5%, X
= 23.5% and Xsd = 205%
At full load
IB[245.4] In Iz
which is satisfactory.
I2[600 to 800] 1.45Iz
244 HANDBOOK OF ELECTRICAL ENGINEERING
which is also satisfactory.
Hence the requirements of IEC60364 are satisfied.
9.5 CABLES WITH ENHANCED PERFORMANCE
during fault situations and these are generally grouped into three types: Sub-transient effects.
Steady state (or synchronous) effects.
For installations that have self-contained power generation plants, e.g. offshore platforms
are also of interest in assessing the fault breaking duty of the switchgear at the times that
to the circuit breaker clearing times e.g. 0.08 to 0.2 seconds. The long-term steady state
is of little concern, unless the system is fe
Escape lighting and signs
Fire and gas systems
Fire extinguishing systems
Fire pump cables
HVAC fire dampers and
Internal wiring in switchboard,
Intrinsically safe systems
(R2 + 2L2)
= tan1 _L
_ = tan1 _X
X = L the inductive reactance.
The exponential term has its maximum positive value when equals /2 radians.
Therefore the maximum value occurs when = /2.
FAULT CALCULATIONS AND STABILITY STUDIES 283
The starting impedance Zms of the motor
Starting to running current ratio
= 0.52 ohms/phase at a power factor of 0.45
c) Find the conductor and armouring impedances for various cables that may be suitable.
From Tables 9.23 and 9.24 the i
With = 0 this current becomes
(1 cos t)
The sinusoidal term in the brackets oscillates between zero and +2.0. This term is called the
doubling factor when the time is t = / has the value of 2.0.
The bad cases occur when the resistance is small, see
10.4 HAZARDOUS AREA ZONES
In the European and UK standards the term Zone is used for hazardous areas, whereas the
Division is used in the USA. In practice the end result of selecting appropriate equipment for
or Division is usually very simila
250 kVA, 4.5% impedance, transformer. Assume an X/R ratio of the transformer of 10.0. The
running efficiency at full-load is 92% and its power factor is 0.85. The starting to running
ratio is 7.0, and the starting power factor is 0.45. The c
0 General requirements, plus Amendments.
1 Construction and verification test of flameproof enclosures of
electrical apparatus, plus Amendments. Note that the type of
protection d is incorporated into this part.
2 Electrical apparatus, type of protection
used for Zone 2 areas near to a Zone 1 area if the equipment are small items e.g. junction
lighting fittings, instrument casings, and local control stations.
10.5 TYPES OF PROTECTION FOR HAZARDOUS AREAS
Most electrical equipment consists of live or
228 HANDBOOK OF ELECTRICAL ENGINEERING
Figure 9.5 Cut-off and prospective current of fuses.
For the selection of fuses the prospective symmetrical rms value of the off-set fault current
Ifrms is calculated as,
Ifrms = Ifpka
= 52,204 amps
The switchboard must be specified to withstand this peculiar situation and it is the task of
engineer to investigate the possibility of it taking place, see sub-section 7.2.11. The
factor that determines whether or not it takes place is t