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Type D MCB 5 Second Tripping Current

Former Community Member over 4 years ago

In BS 7671 17th Edition A3 table 41.3 for type D MCBs gives a 5 second tripping current as 230 x 0.95 / (10In).

When cross referencing time current graphs for type D MCBs I do not get a value of 10In for a 5 second tripping time.

For example see ABB time current graph below giving a current of around 16In for a 5 second trip time.

What is the reason for the difference in values when comparing type D MCB time current charts and table 41.3 in BS 7671 17th Edition A3 for 5 second trip times?

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0 Former Community Member over 4 years ago

That's about the size of it Andy, Mike

See below, the various aspects of Table 7 I posted above

8.6 Automatic operation
8.6.1 Standard time-current zone
The tripping characteristic of circuit-breakers shall be such that they ensure adequate protection of the circuit, without premature operation.
The zone of the time-current characteristic (tripping characteristic) of a circuit-breaker is defined by the conditions and the values stated in Table 7.
This table refers to a circuit-breaker mounted in accordance with the reference conditions (see 9.2) operating at the reference calibration temperature of 30 °C, with a tolerance of C 0 + 5 °.
Compliance is checked by the tests specified in 9.10.

Basically, the manufacturer then defines the time current curve and publishes those curves

The only curves in the standard are those for discrimination purposes depending on if the MCB is a current limiting (zero crossing point) or not

The message is use Type D MCB's at your peril

You might be shocked at the requirements for Type B and Type C
Type B - 3 x In:

0,1 s < t < 45 s
(for In ≤ 32 A)
0,1 s < t < 90 s
(for In > 32 A)
Type C - 5 x In

0,1 s < t < 15 s
(for In ≤ 32 A)
0,1 s < t < 30 s
(for In > 32 A)

Regards

OMS
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0 Former Community Member over 4 years ago

That's about the size of it Andy, Mike

See below, the various aspects of Table 7 I posted above

8.6 Automatic operation
8.6.1 Standard time-current zone
The tripping characteristic of circuit-breakers shall be such that they ensure adequate protection of the circuit, without premature operation.
The zone of the time-current characteristic (tripping characteristic) of a circuit-breaker is defined by the conditions and the values stated in Table 7.
This table refers to a circuit-breaker mounted in accordance with the reference conditions (see 9.2) operating at the reference calibration temperature of 30 °C, with a tolerance of C 0 + 5 °.
Compliance is checked by the tests specified in 9.10.

Basically, the manufacturer then defines the time current curve and publishes those curves

The only curves in the standard are those for discrimination purposes depending on if the MCB is a current limiting (zero crossing point) or not

The message is use Type D MCB's at your peril

You might be shocked at the requirements for Type B and Type C
Type B - 3 x In:

0,1 s < t < 45 s
(for In ≤ 32 A)
0,1 s < t < 90 s
(for In > 32 A)
Type C - 5 x In

0,1 s < t < 15 s
(for In ≤ 32 A)
0,1 s < t < 30 s
(for In > 32 A)

Regards

OMS
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Vote Up 0 Vote Down

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Type D MCB 5 Second Tripping Current

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