Power Management

IEC 60947-2 the “all-risk” insurance for circuit breakers – Part 2

IEC 60947-2 obviates risk and ensures safety and reliability. Its requirements have grown in number to keep pace with evolving technologies and reliability needs of electrical distribution.

COM-POWER-CH19ENCircuit breaker categories A and B

The standard sets out two main circuit breaker selectivity categories: A and B.
Category A: These circuit breakers are typically miniature circuit breakers (MCB) and molded case circuit breakers (MCCBs). They operate at the socket outlet end of final distribution. In the event of a short circuit they trip immediately.
Category B: These circuit breakers have withstand breaking capability. They don’t necessarily trip in the event of a short circuit, which allows downstream circuit breakers to switch off. Their place is in incoming switchboards.

Icu and Ics: essential to operational reliability

Across both categories, circuit breakers must deliver reliability. The two IEC 60947-2 requirements which ensure that reliability are:
• Ultimate breaking capacity (Icu)
• Service breaking capacity (Ics)
Icu is the maximum short-circuit current that a circuit breaker can break without damage. That maximum might be 6,000 or 10,000 amps. Or when it comes to MCCBs, as high as 200kamps. However, in a sense, Icu is essentially as statement of quality for a specific voltage rating.
And that’s where Ics comes in. It is expressed as a percentage ratio of Icu and tells you the maximum short-circuit current if a circuit breaker can break three times and still resume normal service. The higher the lcs, the more reliable the circuit breaker.

Suitability for isolation: seeing is believing

Among the requirements that must be visible even on an installed circuit breaker is suitability for visible isolation. It is particularly important. If a circuit breaker is turned off, it should indicate so visibly. It should not be able to indicate otherwise if the contacts are not open. In other words, it offers proof of isolation.

Positive contact indication. The independent mechanism (green strip) shows if the circuit is fully isolated

Positive contact indication. The independent mechanism (green strip) shows if the circuit is fully isolated

Pollution degrees: should be in all specifications

A requirement that I think is particularly important point and is not often found in general specs is degree of pollution. It determines in what kind of environment circuit breakers can be installed. In a building where there is no dust no humidity, the circuit breaker is comfortable. But in an outdoor public installation, for example, or manufacturing facility there’ll be dust. It might cause leakage currents, affect electrical properties, and lead to dangerous arcs.
I say a useful rule of thumb for household applications it that devices should be pollution degree 2. In my opinion, it’s the right degree for low-pollution non-conductive applications. For more heavy-duty applications, like incoming switchboards for utilities and factories, pollution degree 3 is mandatory. It means circuit breakers withstand conductive pollution dust, humidity, you name it.

Conformity assessment and certification, independent and indispensable
IEC 60947-2 sets out many more requirements. I’ll be running through them in Part 3. But there’s a point I’d like to emphasise first.
It is that all circuit breakers should be conformity assessed and certified compliant in independent, highly reputed third-party laboratories. The standard actually sets the conformity tests – very close to operating conditions – that circuit breakers should undergo.

> See IEC 60947-2 the “all-risk” insurance for circuit breakers – Part 1

> See IEC 60947-2 the “all-risk” insurance for circuit breakers – Part 3


10 Responses
  1. Sunil Kr Dalakoti

    Nice. But I would like to know about Icw mentioned along with two other rating described above. Also the risk assement w.r.t. temp. rise during fault condition?

    Reply
    • Vladislav Tverdochleb Vladislav Tverdochleb

      Hello dear Sunil Kr Dalakoti,
      The Icw, is the short time withstand current characteristic, used in B selectivity category barkers. Usually it is air circuit breakers with nominal current 800-6300A. The Icw is the prospective current during trip delay of circuit breaker without repulsion of contacts. The time delay set in this type circuit breakers is usually given between of 0,05 to 1 second.
      More details on the point of the Icw you can find by following links:
      http://www.electrical-installation.org/enwiki/Circuit-breaker_(_full_page_)

      Reply
  2. Abdul Whab

    Dear i have a bit more confusion about Ics (service rated capacity). Can you please elaborate more?

    Reply
    • Vladislav Tverdochleb Vladislav Tverdochleb

      Dear Abdul, let me be short:
      For the Icu (ultimate SC breaking capacity) the test sequence of operations is O – t – CO (one trip with prospective short circuit value).
      For the Ics (service SC breaking capacity) the test sequence of operations is O – t – CO – t – CO.
      Isc is usually expressed as value of prospective breaking current in percentages (25%,50%, 75%, 100%) of Icu. For example:
      If Ics = 15kA and Ics=100% Icu, it means that device can break ultimate short circuit 15kA more than one time.
      If Ics = 15kA and Ics=75% Icu, it means that device can break without damage 15kA and more than one time if short circuit value is approx. 10kA.
      As higher percentage, as more reliable device to withstand short circuits.

      Reply
  3. Abdul Whab

    —–> it means at 75% of Icu the device will protect only once the max 15KA short circuit and after that the breaking capacity of the device will reduce to 75% Icu till the end of life.
    —–> how we will calculate the Icm value of a particular circuit breaker and why this value not written on the circuit breaker? what is the reletion of Icm with the power factor?

    Reply
    • Vladislav Tverdochleb Vladislav Tverdochleb

      Dear Abdul,
      1. It means that MCB can disconnect circuit @15kA at least once and will be suitable for further use and @10kA at least 3 times and also remain suitable for use. Usually CB’s can withstand more breaking circles and designers (specifiers) shall keep the safety margin for breaking capacity of the breaker vs estimated short circuit.
      2. Icm (short circuit making capacity – when CB is closing in short circuit conditions) according standard shall be not less than Icu and multiplied by coefficient (n) linked to power factor, e.g.:
      PF=0,7 => n=1,5
      PF=0,5 =>n=1,7, ect.

      Reply
  4. Abdul karim

    Hi,

    as per the article above, if I have a LV switchgear with fault level of 44KA for 3 sec, what will be the Icu of outgoing MCCB.
    I proposed to have MCCB with 50KA Icu & Ics , however, the consultant is asking to have 85KA since he considered the 50KA as Icw & he converted the 44KA/3Sec to 85KA/1sec.
    I advised him that MCCB don’t have Icw ( it is only for Cat B-ACB incomer) and 50KA Icu & Ics of MCCB will be enough to break the circuit with 44KA without damage

    kindly advise who is right

    regards

    Reply

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