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CoordinatedProtectionfor Critical Environments

There’s no doubt that a suddenpower failure can have a dramaticeffect on business, especially in afacility with critical operations.Isolating a fault condition to thesmallest area possible is essential inproviding the most reliable electricalsystem with maximum uptime forthe facility. But expensive electronicdistribution protection equipmentmight not be worth the extra costunless a proper protective devicecoordination study is provided by anexperienced engineer.

A properly coordinated system willlimit a fault to the nearest upstreamprotective device. The key is a one-line diagram of an electrical system.Once this diagram is completed andthe brands and models of the protec-tive devices have been selected, thena protective coordination study canbe undertaken.

There are several parameters thatcan be selected for each protectivedevice. For example, the total num-ber, type and sensitivity of the set-tings for each type of device willdepend on the specific device.Adjustment of these parametersallows for what is referred to ascurve shaping.

Curve shaping allows better coor-dination between upstream anddownstream overcurrent protectiondevices. At right is a list of commonpossible parameters.

But, as previously mentioned,before beginning a coordination

Expensive overcurrent protection for electrical distribution

systems might not be worth the expense without a device

coordination study.

Common Device Parameters

● Overload region. (Long Time per Unit). This is the long-time trip setting of the over-current protective device. This parameter, alsoknow as continuous amperes, is a percentageof the breakers nominal rating and can typi-cally be set at 20% to 100%. This setting isusually achieved with a thermal overload in amolded-case circuit breaker.

● Long-time delay.This setting allows for inrush from motors topass without tripping the breaker. The settingaffects the position of the “I squared T” slopejust below the continuous current setting.

● Short-time pick up. This is typically provided with an adjustment of5 to 10 times. This setting allows downstreamovercurrent protection devices to clear faultswithout tripping upstream devices and canalso be adjusted to allow for transformerinrush current.

● Short-time delay and instantaneous override.

This setting postpones the short-time pickup,and can be done on a fixed setting or an Isquared T ramp setting. This allows for better

coordination between upstream and down-stream devices. An instantaneous override canbe set at high current to override this functionand protect electrical equipment. The I squaredT function of the short time delay can providebetter coordination when coordinating abreaker with a fuse.

● Instantaneous.This setting will trip the overcurrent protectivedevice with no intentional delay.

● Ground-fault setting. (Ground Fault per Unit).

This is the percentage of the rating of thebreaker for the ground fault setting. Per theNational Electrical Code, ground fault cannotexceed 1,200 amps, regardless of the size ofthe breaker.

● Ground-fault delay. This setting allows for a time delay beforeground-fault pickup. This allows for betterselective coordination between multiple levelsof ground-fault protection. In addition, thetime delay cannot exceed one second (60cycles) for ground-fault currents of 3,000amps or more.

BY KEITH LANE, P.E.Lane Coburn and Associates, LLC.

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study, the engineer might need todesign a one-line diagram—if onedoesn’t exist already—of the buildingelectrical systems, and coordinatewith the electrical contractor,equipment provider, or both, todetermine the actual equipment to beinstalled. The following is requiredfor an accurate protectivecoordination study:

1. Description, make and cata-logue numbers of protectivedevices.

2. Full load current at the protec-tive device.

3. Transformer kVA rating, imped-ance and inrush data.

4. Available fault current at theprotective device.

5. Conductor cable informationincluding current carryingcapacity and insulation type.

6. Protective device design require-ments from the serving utility.

These days, it’s common toperform complicated electricalprotection coordination studieswith the help of advancedcomputer software. Thesesoftware platforms willtypically contain librariesthat include requiredsettings for most of thecommon overcurrentprotective devices.

And as noted above,with the review ofprotective coordinationstudy basics, the reliabilityof an electrical system andits equipment can only beassured if propercoordination isimplemented among allthe various protectivedevices.

THE NEC MANDATEThere are instances where theNational Electrical Code (NEC)requires a protective coordinationstudy. There are also times when K-rated transformers employed to dealwith electronics and non-linear loadscan reduce reliability if not properlycoordinated.

On a typical transformer, thecurrent and associated magnetic fieldis 90 degrees out of phase with thevoltage. When you close a breakerand turn on a transformer, theinstantaneous magnetic field can betwice as high as normal. In the“ideal” transformer, the currentrequired to supply this magnetic fieldwould also be twice as high.However, in a real transformer thecore is saturated and the actualcurrent required to create the fieldcan be 12 times as high as normal.Factors such as the size of the coreof the transformers and the timethe voltage is applied play rolesin determining the amount ofinrush current.

The actual inrush currentmentioned above would be differentbased on the actual transformermanufacturer. It is critical for theconsulting engineer to contact thespecific manufacturer of the

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Figure 1

For information enter 412


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transformer supplied in the field. Ifactual transformer inrush data is notknown, common industry standard isto assume the inrush is 12 times for0.1 seconds and 25 times for 0.01

seconds. Figure 1 (p. 30) illustratesthe transformer inrush at 12 timesfor 0.1 seconds.

Engineers were running intotrouble some years back when the

K13-rated transformerwas becoming moreprolific in regularoffice environments. AK13-rated transformeris oftentimes just alarger transformerwith a smaller ratingto compensate forharmonics. The same110-amp breakertypically on theprimary side of aregular 75-kVAtransformer may tripwhen protecting a 75-kVA, K13 transformer.For sizing of the

primary side, overcurrent protectivedevice for K13 or higher trans-formers, I recommend multiplyingthe input full load amps of atransformer by 125% and going tothe next common size up. Inaddition, a breaker with theinstantaneous setting is oftenrequired to allow for the transformercurrent inrush. As a final step, Irecommend a coordination study toensure the system will work before itis too late, after construction iscomplete and the engineer is stuckwith an angry owner.

Selective coordination is requiredwhen more than one elevator issupplied by a common feeder, perNEC 620-62. Figure 2 (above) is anexample of a coordination studyillustrating feeder breakerovercurrent protection, elevator fuseovercurrent protection and elevatormotor start-up curves.

NEC 517-17 requires that if ground-fault protection is provided for theservice disconnecting means,then an additional step of ground-fault protection shall be providedin the next level of feeder dis-connecting means downstreamtoward the load. Figure 3 (above)is a good example of what a properlycoordinated ground fault studyshould look like.

Figure 2 Figure 3



“How can I ensure the compatibility of my UPS with my generator set?”

E-mail: [email protected]

POWER QUALITY

A critical element of the UPS-generator set relationship is compatibility. Too often,generator sets are the wrong size for the UPS, the units are not synchronized or thestart function fails. Any of these problems can leave you with no electricity and nooptions. It is therefore crucial to make the best choice for the most reliable backup power source designed with the greatestcompatibility. To better understand thisrelationship, consider the following:

� Proper sizing: Achieving UPS and generatorset compatibility stems from the need tocorrectly size the generator set based on the nature of the UPS. Some UPS designsgenerate more current harmonics than others,and it’s those harmonics which necessitateoversizing the generator (to avoid the effect of capacity-robbing heating which occurs as a result of the harmonics). UPS designs whichminimize harmonics may ultimately prove themost cost-effective when all elements of thepower quality system, including the generatorset, are considered.

� Synchronized controls: A UPS control method with no filters and low harmonicdistortion is more easily synchronized with a generator set’s controls. UPS units that usea rectifier/charger input control method caninduce total harmonic distortion (THD), createexcessive heating and cause numerousproblems with the generator set controls.

� Recharging the backup: Fast, reliable rechargeof the backup power source is a must. A UPSpower source, such as kinetic energy, that isnon-degrading and requires a low rechargecurrent will provide the most dependableamount of recharge in the shortest amount of time.

� Generator set start reliability: A generator setstarting option can ensure a sufficient amountof starting power to alleviate the problemsassociated with unpredictable power sources,such as a degraded set of batteries.

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NEC 230-95 indicates that all 480-volt 3-phase services 1,000 amps andabove must be installed with aground-fault relay. The setting of theground-fault relay cannot exceed1,200 amps, regardless of the size ofthe overcurrent protection device. Inaddition, the time delay cannotexceed one second (60 cycles) forground-fault currents of 3,000 ampsor more. There shall be a minimumof six cycles (0.1 second) ground-fault delay between ground-faultdevices in health-care facilities.

Ground-fault settings for mainbreakers serving downstream motorsthat are set too low or too fast maytrip a main overcurrent protectiondevice before tripping the localthermal magnetic overcurrentprotection device during motorstarting ground faults. On the otherhand, ground-fault settings that aretoo high can cause undue damagebefore a ground fault is interrupted.It is important to provide the ground-fault setting that will not permitnuisance tripping, but will protect

the electrical equipment fromexcessive damage during an event.

In my experience, sometimesperfect coordination between a set ofdevices cannot be obtained. Certainsettings may be required on a breakerthat could affect the settings of manybreakers. In some cases, there maybe many levels of breakers that maycause overlap of the breaker curveswithin the tolerance of the curves. Itis at these times that experience willallow the engineer to make judgmentcalls as to certain compromises incoordination between devices. Theengineering behind protectivecoordination studies is not an exactscience, by any means.

On many occasions, I have seencompleted projects with no protectivedevice study. In such cases thebreaker manufacturer will ship thebreakers with all settings set to themost sensitive. This will ensure themost protection, but will increasefalse trips and is typically not goodfor the reliability and uptime of thesystems.

As soon as the owner complains ofa false trip, the facility personnel willprobably set all of the dials to leastsensitive. This reduces the likelihoodof false trips, but might notadequately protect the electricalsystem and reduces selectivecoordination of the system.

In short, a coordination study istypically required to ensure that themost reliable electrical system hasbeen installed. In addition, there areinstances where the NEC requiresthat a study be performed. In eithercase, the cost of a coordination studyis cheap insurance for most anyfacility that would be adverselyaffected by an extensive poweroutage.

The important point to rememberis that protection equipment can bean expensive investment, and it’swell worth the cost of investing in acoordination study by an experiencedelectrical engineer. ��

What’s in aCoordination

Study?

In addition to understanding thebasics of coordination studies, otherkey issues involved include:

● Proper sizing of the transformerprimary breaker based on trans-former saturation and the prolifer-ation of K-rated transformer.

● Elevator protection coordinationas required per the NEC.

● National Electrical Code article517 and ground fault coordinationstudies.



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