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Topic Title: Current Limiting MCCBs & Cascading
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Created On: 23 March 2019 08:44 am
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 23 March 2019 08:44 am
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Selk

Posts: 29
Joined: 21 April 2018

How does current limiting MCCBs help reducing the interrupting capacity of downstream breakers?

Say, if the upstream current limiting MCCB trips before the peak of the sine wave, for the fault down of the downstream MCCB, then there is no coordination.

Is there some inbuilt impedance in the current limiting MCCBs which reduces the fault current?

How is the cascading/discrimination achieved?
 23 March 2019 08:51 am
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Zoomup

Posts: 6117
Joined: 20 February 2014

The cable impedance may reduce downstream fault current magnitudes. Do M.C.C.B.s trip off in less than a quarter of a cycle?

A brief overview of M.C.C.B.s.

http://www.linkedin.com/pulse/...c-knowledge-lily-chan

Z.
 23 March 2019 09:04 am
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Zoomup

Posts: 6117
Joined: 20 February 2014

How is the cascading/discrimination achieved?


A tool is available.

http://www.youtube.com/watch?v=gXaJMnXGbjo

Z.
 23 March 2019 09:15 am
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Zoomup

Posts: 6117
Joined: 20 February 2014

More in depth stuff relating to trip times.

http://www04.abb.com/global/se...aper%2BVolume%2B1.pdf

Z.
 23 March 2019 09:54 am
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mapj1

Posts: 12039
Joined: 22 July 2004

There is no current limiting resistor, what is actually being limited is the fault duration, and as it is current squared times time that does the burning and melting during a fault, a short duration large fault is equivalent in damage to a longer duration but lower fault current.

Pictures showing a nice sinusoidal current being interrupted before it meets the peak, are not quite right - faults can occur anywhere in cycle, including at the peak, and then you are at the mercy of circuit impedances, until the fuse or breaker starts to open, and in reality fault waveforms are quite odd.

-------------------------
regards Mike
 23 March 2019 10:06 am
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Zoomup

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Joined: 20 February 2014

Please have a look at 40 minutes in to this video about discrimination of M.C.C.B.s.

http://www.youtube.com/watch?v=uUgGlwStbtw

Z.
 23 March 2019 12:27 pm
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AJJewsbury

Posts: 17795
Joined: 13 August 2003

How does current limiting MCCBs help reducing the interrupting capacity of downstream breakers?

Say, if the upstream current limiting MCCB trips before the peak of the sine wave, for the fault down of the downstream MCCB, then there is no coordination.

Is there some inbuilt impedance in the current limiting MCCBs which reduces the fault current?

How is the cascading/discrimination achieved?

There will be some impedance in any protective device (enough to produce enough resistive heating to vapourise copper in the case of a fuse, or from the magnetic and thermal elements in circuit breakers) - usually small enough to be ignored in most calculations - but perhaps enough to be slightly significant under fault current conditions.

I gather than some types of MCCB allow their contacts to partially open during a a fault - so limiting the fault current while giving any downstream devices the opportunity to open fully. If the fault isn't cleared within the MCCB's delay time then its contacts fully open. But I suspect that approach is the exception rather than the usual rule.

In general though, where back-up protection kicks in then you loose selectivity (because the upstream device opens as well as or instead of the downstream device). But it's not an all or nothing approach. The value of an actual fault current will vary tremendously depending on where in the circuit the fault occurs. If at the end of the circuit the fault current will be much lower than for faults right next to the protective device.

Ideally you'd have complete selectivity and every device being fully rated for the maximum possible fault current at the position it's installed, however economics and practicality can interfere with that. Most UK domestic supplies have a deemed maximum fault current of 16kA at the intake - but just try finding a MCB with a 16kA breaking capacity (let alone at a reasonable price). So typically 6kA devices are installed, together with a (max) 100A fuse upstream. What that means is for faults up to 6kA the MCB should open first and selectivity is achieved - above 6kA the fuse might open first (and so selectivity is lost) - but that's only going to happen with a very small proportion of faults - i.e. ones very close to the consumer unit.

Remember that nothing in BS 7671 demands selectivity under all conditions - although always desirable - it's only actually required when needed for safety.

- Andy.
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