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

Vladislav Tverdochleb | July 16, 2013 | 19,901 viewseMail 580 inShareTweet

IEC 60947-2 has been around a while. Originally called IEC 947-2, it got its current name way back in 1997. So, you may wonder, why talk about it now?

Let’s say a timely reminder is about due.

Substandard circuit breakers galore

Electrical distribution has become an ever more important part of our daily lives. More and more people worldwide are enjoying comfortable, device-driven lifestyles, while business and industry in developed and developing economies are dependent on ICT and always available power.And if electrical distribution pervades every aspect of work, play, and travel, so, too, do circuit breakers. But not all circuit breakers qualitatively comply with IEC 60947. And in fact, today’s deregulated, fast-changing global market is flooded with billions that don’t. That spells trouble for end-users – both domestic and industrial.

Circuit breakers the basic building block of electrical safety

Certified, conformity-assessed, standard-compliant devices last decades. circuit breakers in switchboards operate for 10 to 15 years at a time, for example. Low-price, substandard – rather, standardless! – circuit breakers may work for a while. But not much longer. When – rather than if – they fail, the cost may be high. Money, time and – above all – lives may be lost.Lest we forget: the circuit breaker affords essential protection against short-circuits and overcurrent. They prevent fire.In fact, I like to call circuit breakers the basic building block of electrical protection. Of course, some countries also require other kinds of mandatory safety devices. Devices that prevent and protect against earth leakage, electrocution, and the damage caused by surges.

The humble circuit breaker

Compounding the problem of the market flooded with substandard circuit breakers is one of ignorance of, and even disregard for, the humble circuit breaker.End users may be forgiven for not caring too much about the technical niceties of circuit breakers. I admit, it’s not their concern even though it might eventually become their problem. Design engineers and general contractors, however, should have in-depth knowledge of circuit breakers and standard IEC 60947 2, which governs their specifications.And if the tech specs are clear and standards-based, then end-users, too, will benefit. They’ll know what their circuit breaker is supposed to do and what it does do.

Circuit breaker casings tell you what they do

Clearly, then, it is critical to select the right circuit breaker. The right choice is a reliable one that should be fit not just for general purpose, but should also meet the specifications for the application in hand.

Modular Circuit Breakers marking examples basing on requirements in IEC/EN 60947-2

Molded case circuit breakers (MCCB) marking examples basing on requirements in IEC/EN 60947-2

IEC 60947 2 requires circuit breakers to have information printed on the outside. The more there is, the better the device, and the more you will know about what it does. It is information like:• Rated current (should always show, even when the circuit breaker is installed)• Manufacturer’s trademark• Compliance with standard IEC/EN 60947-2• Selectivity category• Rated voltage• Rated impulse withstand voltage• Rated short circuit making capacity• Rated short-circuit breaking capacity• Reference temperature (if different from 30°C)• Pollution degree• Rated insulation voltage• Suitability for isolation (should show even when circuit breaker is installed)• Clearly marked on and off positions (should show even when circuit breaker is installed)

This information is indicated by abbreviations and diagrams. IEC 60947 2 spells it out in words.

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

Vladislav Tverdochleb | July 26, 2013 | 15,109 views

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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.

Circuit 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

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 indispensableIEC 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

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Tags: Circuit Breakers, IEC, IEC 60947

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

1. kulkarni shirishAugust 20th, 2013

Intersted

Reply

2. Sunil Kr DalakotiAugust 24th, 2013

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

o Vladislav TverdochlebSeptember 11th, 2013

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

3. AbdelfattahSeptember 3rd, 2013

great , thanks so much and waiting more.

Reply

4. Abdul WhabDecember 30th, 2013

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

Reply

o Vladislav TverdochlebJanuary 2nd, 2014

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.

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5. Abdul WhabJune 26th, 2014

—–> 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

o Vladislav TverdochlebJuly 2nd, 2014

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=1,5PF=0,5 =>n=1,7, ect.

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

Vladislav Tverdochleb | August 8, 2013 | 6,288 views

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This is the third part of my blog on selecting a circuit breaker. And there are still some more IEC 60947 2 requirements to consider.

Reference voltages

One I reckon particularly important requirement is rated insulation voltage (Ui). Ui is the voltage that references the voltage environment. In other words, the higher the Ui the less damage is likely. In daily use, 220 volts is safe. But if a circuit breaker is tested at the higher voltage rating of 500 volts, you can be sure its isolation properties will not be changed when the circuit breaker is in use.The rated impulse withstand voltage Uimp sets the maximum voltage a circuit breaker can withstand without failure. It is expressed in kilovolts (kV). In an apartment, for example, 4kV is ample, while industrial devices have Uimp of up to 8 kV.

Reference temperature

Another reference value is the reference temperature. It is the temperature for which a circuit breaker is calibrated. For household applications it is usually 25°C or 30°C. This not enough for industrial applications where the temperature in a switchboard can reach 50°C and higher. However, manufacturers usually supply temperature rating tables which show the nominal current for different temperatures. For example for a 63-amp circuit breaker, it may be 70 amps at 30° and 56 amps at 70°C.I think we have covered most of the important requirements in IEC 60947 2. But there are still one or two more points to raise on the subject of selecting a safe, reliable, robust circuit breaker.

Extra useful features for circuit breaker selection

For example, a circuit breaker should incorporate a lockout mechanism to prevent any unpermitted closing. It stops both manual and remote closing, and only releases the circuit breaker when a safe power source is used.And à propos of manual and remote, a valuable feature is a trip indicator that can be mounted on the trip unit. It tells technicians whether the circuit breaker tripped because of overload or a short circuit or whether it was manually turned off.

MCB locking mechanism preventing accidental switching

 

Trip indication reduces fault identification time

Some requirements make it a good idea to have auxiliary systems like remote control and monitoring. Remotely controlled reclosing mechanisms, for example, provide backup in the event of accidental or harmless tripping.

In this three-part blog we have run through the most important requirements from IEC 60947-2. You should bear them in mind when selecting a circuit breaker, although not all are needed by all applications.But if you think I’ve missed anything, if you have any tips, or just want to know more about IEC 60947 2 put your comment.