MB30-CS datasheet(72/105 Pages) MITSUBISHI

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Table 6.4 Allowable Fault Conditions in Conductors

6.6 Protective Coordination with Wiring

6.6.1 General Considerations

If it is assumed that the heat generated by a large

current passing through a wire is entirely dissipated

within the wire, the following expression is applicable

(for copper wires):

(

t=5.05!10 log

234+T

234+To

)

: Current(A, rms)

: Wire cross-sectional area(mm2)

: Current let-through time(sec)

: Wire temperature due to short circuit(

°C)

To : Wire temperature before short circuit(

°C)

Assume that short-circuit current occurs in a wire car-

rying its rated current (hot state To=60

°C). If 150°C is

the allowable temperature T, the following expression

is applicable (see also Fig. 6.13):

Allowable short-circuit

current accoeding to

I2t

kA, sym. (PF)

1.5

2.5

120

150

185

240

Allowable

I2t

A2!sec

Wire size

mm2

Notes: 1. Allowable

I2t is calculated assuming that

all heat energy is dissipated in the

conductor, conductor allowable maximum

temperature exceeds 150

°C, and hot

start is applied, at 60

°C.

Is is an asym. value of allowable short-

circuit current reduced to below the

allowable

I2t, assuming half cycle

interruption for 16mm2 or less and one

cycle interruption for 25mm2 or more.

Allowable

I2t=14000S2

Considering let-through energy (

∫i2dt) in a fault where

the protector has no current-limiting capability, if short-

circuit occurs when let-through current is max.,

∫i2dt

is:

where current le is the effective value of the AC com-

ponent. Half-cycle interruption is applied to wire of up

to 14mm2, and one-cycle interruption to larger wires.

Table 6.4 is restrictive in that, e.g., in a circuit of fault

capacity of 5000A or more, 2.5mm2 wires would not

be permitted. In practice, the impedance of the con-

ductor itself presents a limiting factor, as does the in-

herent impedance of the MCCB, giving finite let-

through

I2t and Ip values that determine the actual

fault-current flow.

6.6.2

600V Vinyl-Insulated Wire (Overcurrent)

Japanese Electrical Installations Technical Standards

(domestic) specify vinyl-insulated wire operating tem-

perature as 60

°C max., being a 30°C rise over a 30°C

ambient temperature. This is to offset aging deterio-

ration attendant on elevated temperatures over long

periods. Criteria for elevated temperatures over short

periods have been presented in a study by B. W. Jones

and J. A. Scott (“Short-Time Current Ratings for Air-

craft Wire and Cable,” A

IEE Transactions), which pro-

poses 150

°C for periods of up to 2 seconds, and 100°C

for periods in the order of 20 seconds. These criteria

can be transposed to currents for different wire sizes

by the curves given in Fig. 6.14. Such figures, how-

ever, must be further compensated for the difference

between vinyl materials used for aircraft and for

0.014!106

0.032!106

0.088!106

0.224!106

0.504!106

1.40!106

3.58!106

8.75!106

17.2!106

35.0!106

68.6!106

126!106

202!106

315!106

479!106

806!106

1.17 (0.9)

1.76 (0.9)

2.93 (0.9)

4.68 (0.9)

6.79 (0.8)

10.5

(0.6)

16.0

(0.5)

17.3

(0.3)

24.2

(0.3)

34.5

(0.3)

48.3

(0.3)

65.6

(0.3)

82.8

(0.3)

103

(0.3)

128

(0.3)

166

(0.3)

Fig. 6.13 Temperature Rises Due to Current Flow in Copper Wires

×103

×104

1000

700

500

300

200

100

44 5 6 7 8

(A/mm2)2·sec

Approx.

(A ·sec) in

1 cycle interruption

(Power factor is 0.5.)

Approx.

(A ·sec) in 1

cycle interruption

(Power factor is 0.3.)

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