Ideal Air Circuit Breaker

Preference for an Air Circuit Breaker(ACB) can be assessed based onseveral parameters. With diverseapplications and system configurations,the features offered by the ACB haveto be really exhaustive. This articleattempts to cover most of the majorindustrial requirements, by classifyingthem into ten categories. They are:

1. Safety:

‘Safety First’ has always been theprinciple of every industrial applicationwhere a human is expected to interactwith machine.

In case of ACB, degree of interactionhas always been higher – be it normaloperation, routine maintenance or lifecycle enhancement. Therefore,operational safety is a very crucialparameter for an ACB.

While ACBs have ensured safeoperation and maintenance over thepast decades, industry feedbackhighlights some more aspects whereuser's safety needs to be ensured.

With increasing preference to controlroom concept, use of electricallyoperated ACBs is also increasing.When a breaker is remote controlled,there is always a risk of remotecommand closing the breaker when themaintenance person is racking it in/out. To prevent such a harmfulsituation, breakers should have a safetyinterlock, whereby the remote closingcommand will be blocked during ACBracking.

It is quite possible that aftermaintenance, the operator forgets toinstall the arc chutes or secure themproperly. If the breaker clears a fault inthis condition, it would cause severedamage. Hence, the breaker should notclose if all the arc chutes are notsecured properly.

The breakers should be equipped withsafety shutters, with independent

padlocking of top or bottom side, toensure safety during maintenance.

2. Panel design:

When ACBs of different frame sizesare mounted in one panel, uniformity inbreaker design across the frame sizesplays an important role. If the breakershave common depth and height for allthe frame sizes, it would make easythe link work design and avoid wastageof valuable panel space.

Also, uniform location and commondimensions of panel cut-out for theACBs in various frame sizes will addto the aesthetic value of the panel.However, attention to aesthetics shouldnot result in wastage of panel space.This is only possible with right or leftaligned breaker facia, a feature grosslyoverlooked.

Given the constraints on panel spacein most of the industry premises, thereis an increased pressure on makingcompact panels. By virtue of being thebiggest LV switchgear equipment, ACBplays an important role in arriving atthe panel dimensions. Therefore, acompact ACB is a welcomeimprovement.

However, compactness should not beat the cost of functional parameterssuch as breaking capacity andtemperature rise. A compact ACB

should also be operable at the ambienttemperatures in tropical countrieswithout derating. Inside the panel,temperature (micro ambient) is alwayshigher than outside (macro ambient)ambient by around 12-15 degrees C.Hence, when the manufacturers specifybreaker technical data for insideambient, it should be compared withequivalent outside ambient whileworking out ACB derating.

Making a breaker compact generallysacrifices the size of the terminals. InIndia, panels are designed withaluminium termination, which requireswider terminal size and the ACB shouldbe able to support it without adapters.

No terminal design can directly be usedfor terminating all the possible linkconfigurations. Some terminal designsare easier to manufacture but call forhigher effort from the user. Typicalexample being horizontal terminals. Ourexperience reveals flat terminals as thebest configuration, though it calls forhigher efforts from the manufacturer.Flat terminals can be, in fact, aptlycalled ‘Universal terminals’.

It is important that ACB manufacturerprovides flexibility of on-site adaptation,without the need for dismantling theexisting terminals.

While most manufacturers claimsuitability of terminating two wires on

Flat terminals offer flexibility and easy access

- Mr. J.S. GangalPowergear Group (ACB)

control terminals, the unspecified desireis actually for ‘independent’ terminationof 2:2.5 mm2 wires.

In most cases, industrial environmentcalls for higher degree of ingressprotection. As front facia is the onlypart to project out of the panel door, itshould have higher ‘intrinsic’ IP level.

3. Performance:

Needless to mention, the breakershould comply with the latest applicablestandards.

ACB holds the ultimate responsibilityfor clearing the fault and protecting thecircuit. It should trip only when thedownstream devices fail to operate sothat healthy feeders elsewhere in thesystem are not isolated unless thereis an emergency. Therefore, the breakershould have adequate ‘short timewithstand capacity (I

cw)’. The previous

issue of L&T Current Trends talkedabout the advantages of Fully RatedSystem, which also reflects on ACBhaving adequate withstand capacity.Ideally, withstand capacity should beequal to I

cu (rated ultimate short circuit

breaking capacity) and Ics (rated service

short circuit breaking capacity) thoughthe standards permit I

cw to be lower

than Ics

. At times, Ics

and Icu

arespecified by the manufacturer with no/vague reference to I

cw. Users must be

careful in making the selection.

4. Protection:

Circuit protection is the main functionof the ACB. Breaker releases offer avery simple, cost effective and reliableprotection. Since release operatesdirectly on the trip mechanism of theACB, it trips the breaker irrespective ofavailability of control supply orhealthiness of trip circuit.

Initially releases were electro-mechanical. They provided the basicoverload and short circuit protectionefficiently and offered a very economicalsolution. They provided effectiveprotection even to cables by usingbimetal thermal memory.

Then came the static release. Theyoffered better reliability and accuracyas compared to the conventional

electromagnetic releases and still gavea cost effective solution. Hence, theybecame very popular all across theuser industry.

Next evolution was micro-computerbased releases. They offered versatilityand higher accuracy in protectionsettings. All these releases offered thenecessary basic protections essentialin the ACB. But today’s complexcontrol circuits require releases:

l that can be remotely programmedand monitored.

l that offer additional currentprotections like undercurrent, currentunbalance, etc.

l that take voltage inputs, wideningthe scope of protection to includevoltage, frequency and power relatedprotections.

l that carry out measurements ofcurrent, voltage, power and severalderived parameters for energymanagement thus reducing severalmetering equipment.

l that offer communication so as tocapture all the metering, faultparameters, etc., at a remotelocation.

l that also monitor total harmonicdistortion and amplitude ofharmonics.

Owing to extensive usage ofelectronics and the release being a verysensitive part in the breaker, whereverrequired, EMI/EMC tests on the releaseare a pre-requisite.

Today, there are specific requirementsfor higher neutral rating with increasingusage of computers/UPS, etc. Withconsiderable zero sequence currentsadding up in the neutral, the requiredneutral rating goes well beyond thephase current, as high as 200%.Special versions of breaker should beavailable for such requirements (without50% derating of higher ratings). Also,releases should be able to protect 200%neutral currents. For normal circuits, arelease should be able to protect 50%and 100% neutral ratings.

There could be numerous other system-specific requirements, which thereleases will have to cater to and the

list ofdesirablesfeatures canbe reallylong. But withelectronics atyour service,nothing isunachievable.

In currentmarket, it isnecessary tohave a rangeof suchrelays andreleasesavailable toselect from,depending onthe systemrequirements and economics.

5. Maintenance:

At the end of electrical life,maintenance man should be able toreplace the breaker poles in theshortest possible time and in the ‘Fixit right the first time’ way to reduceplant downtime and production loss.This convenience generally remainsrestricted to lower ratings of ACBs.

During ACB maintenance, wheneverrequired, replacement of cradleterminals is done from the rear side.However, with the breaker mountedinside a panel, access from the rearside is restricted. Even if one has theaccess, he will have to dismantle theentire cable and link work for replacingcradle terminals. Therefore, it shouldbe possible to carry out replacementof cradle terminals from the front side,preferably in a phase by phase manner.May be at the cost of much highereffort in ACB design, but this featurewill certainly make the ACBmaintenance much easier for the users.

Arc chutes have to be routinely openedby the maintenance man for contactinspection. To facilitate the above, itshould be possible to remove and re-fix the arc chutes without using anytool. This feature becomes all the moreimportant in case of a fixed breaker. Itmust be remembered here that the‘easy-to-remove’ arc chutes must alsowithstand severe stresses encounteredwhile clearing a major short circuit.

Release showing timecurrent characteristics

6. Security:

ACB needs to be secured for preventionof unauthorized access and inadvertentoperation.

Operations susceptible to unauthorizedaccess are ACB racking and opening/closing. ACB trips during rack-outoperation, resulting in disruption ofsupply and hence, racking operationmust be protected against unauthorisedaccess. Similarly, ON and OFF buttonsmust also be guarded against foul play.

Protection settings on the release areextremely important for safety of theequipment downstream. Once set, theyshould not be touched unless a changeis called for. Therefore, access torelease settings must be secured toprevent tampering.

During a typical maintenance shutdown,several breakers are removed from thecradles. While replacing the breakers,one should not be able to inadvertentlyload a breaker of the same frame sizein a wrong cradle.

7. Modular and User-friendly:

Though breaker configurations arenormally ordered as per the systemrequirements, there are severalinstances where users have to add/delete accessories, where modularityplays a vital role.

Though modularity is a commonly usedconcept today, its extent of utilisationis varying. If an accessory can be fittedat site, but takes hours of assemblytime and special skills, is it trulymodular?

Therefore, one should be able to fitACB accessories on-site, without usingany special tools in minimum time. Fora drawout ACB, this should even bepossible without removing the ACB fromthe panel. The control wiring should

also be easy to fix, without the needfor calling a specialised person. Itwould be extremely beneficial if nocontrol termination is required to behandled while replacing accessories.

Though draw-out breakers are verypopular today, there is still a largepopulation of fixed breakers in theindustry. In a plant where a combinationof both types exist, it would bepreferable if a fixed breaker and a draw-out ACB (breaker portion) areinterchangeable.

In a PCC where a large number ofACBs are installed, it is essential thateach ACB identifies itself clearly to themaintenance man, without evenopening the panel door. The front ofthe ACB, which projects through thedoor cut-out should give as muchinformation as possible about theaccessories installed (and notinstalled!!). Even for finding out thecontrol voltage of various accessories,the operating personnel have to referto the drawings, as they cannot openthe breaker every time. Should thebreaker front display this information,it would be quite convenient.

8. Complete range:

A maintenance man always prefersfewer variations in the equipmentinstalled in his plant as standardizationhelps him in product familiarity andlower spares requirement. Therefore,the breaker manufacturer should be ableto provide complete range in terms ofratings, 3/4-pole versions and fixed anddraw-out models.

9. Environment friendly:

Implementation of environment friendlypolicies (ISO 14000) in most of theindustries necessitates use of everyequipment including switchgear, whichis environment friendly in manufacturingand in service.

Environment consciousness of themanufacturer can be seen right fromthe ACB packing made with disposablematerial to avoiding use of harmfulheavy metals. It is lesser known thateven paint pigments can be madeenvironment friendly by avoiding leadcontent.

Another effort towards environmentconservation can also be seen in lifecycle enhancement of the product. Lifecycle can be enhanced through acombination of higher basic life as perthe product design and subsequentparts replacement.

10. “What You Want Is What YouGet”:

ACB caters to a wide variety ofapplications – from one stand-aloneACB to hundreds of ACBs integratedin a modern plant. Extent of intelligenceto be packed in the protection releasealso varies from one application to theother.

As a result, ACB manufacturer has tooffer a wide spectrum of products –from high technology to economy.Standardization in ACB specificationshelps the manufacturer manage thecomplexities and expedite deliveries inthis wide spectrum of products.However, this style of feature packagingconfuses the user.

Therefore, the users must be able toselect the very features that they needand pay for them.

Conclusion:

Any breaker, which meets the aboveexpectations will certainly keep theoperation and maintenance personnelhappy. But, for achieving this, thebreaker must be designed for theUSERS, designed to surpass worldbenchmarks!!!