Ampere Interrupting Capacity Calculations

Transcription

1 Strand Lighting Inc. Ampere Interrupting Capacity Calculations Or Why Are There Extra Fuses In My Dimmer Rack? 23-Apr-2005 Ampere Interrupting Capacity Page 1 of 11 Pages

2 1.0 INTRODUCTION When designing dimmer rack circuit protection for a new or renovated facility, theatrical consultants typically specify a dimmer rack main circuit breaker (or fused disconnect) capable of supporting the expected connected lighting load. Typical Fused Disconnect & Molded Case Main Circuit Breaker This may not be sufficient equipment (or personnel) protection since this device only provides overload protection. It is critical that the theatrical consultant confirm with the project electrical engineer that the overall electrical system design provides the proper interrupt protection to the dimmer rack as well. The short circuit "interrupting" protection rating is different than the normal running ampere rating of the main protective device. The interrupting rating is the maximum fault current that would flow if there were a bolted short (like a wrench falling against some buss bars) in the circuit. The fault current rating is determined by calculating the AIC (Ampere Interrupting Capacity) short-circuit rating of the electrical system. This calculation needs to be performed by a qualified electrical engineer as there are many variables ranging from conductor length, conductor constants, transformer KVA, transformer impedance and the available short circuit value at the beginning of the circuit. Once the fault current rating is known, the interrupt protection requirement should be listed in the project bid specifications or drawings. 23-Apr-2005 Ampere Interrupting Capacity Page 2 of 11 Pages

3 2.0 AMP TRAPS Dimmer rack AIC ratings vary between 5,000 and 100,000 AIC. In order to increase the AIC ratings of dimmer racks manufacturers sometimes add current limiting fuses (commonly known as "Amp Traps") to the buss structure design of their dimmer racks. Amp Traps are special current-limiting (not overload-limiting) fuses that are capable of opening a faulted circuit before the fault current has sufficient time to reach its maximum value. Amp Traps are always used in conjunction with another overload current protection device such as a main circuit breaker, or fused disconnect. Typical Amp Traps 23-Apr-2005 Ampere Interrupting Capacity Page 3 of 11 Pages

4 3.0 DO I NEED AMP TRAPS IN MY DIMMER RACK? Amp Traps installed in dimmer racks are typically optional, and are only added to the dimmer rack assembly if the AIC requirement for the facility is known when the project is bid. The additional buss structure added to a dimmer rack to support Amp Traps will increase the cost of the rack. Before adding them to your specification, be absolutely certain that they are required. Strand recently visited an installation with 16 dimmer racks (with rack to rack bussing) installed on two 2400A services. These particular racks had an AIC rating of 50,000. However the facility had paid additional money to include amp traps in the bill of materials to increase the AIC rating of the dimmer racks to 100,000. Unfortunately, the fault current calculations by the facility electrical engineer showed that even with a relatively large service, the available fault current was only 43,000 AIC effectively rending the Amp Traps unnecessary. Had this calculation been made before the tender, project cost savings could have been achieved. Amp Traps Amp Traps In A Strand Lighting CD80SV Dimmer Rack 23-Apr-2005 Ampere Interrupting Capacity Page 4 of 11 Pages

5 Amp Traps Amp Traps Installed In A Strand Lighting C21 Dimmer Rack 4.0 DIMMER MODULE CIRCUIT BREAKERS A related question that we are frequently asked is how a dimmer rack can be rated for 100,000 AIC - but have a dimmer module branch circuit breaker that is only rated for 10,000A AIC? This is because the circuit breaker must be rated for the applied voltage, continuous amperage load, and must also have an AIC rating equal to or greater than the available current at the location in the circuit where it will be installed. Since the dimmer module is located between the main and branch circuit, a 10,000 AIC breaker is appropriate in many dimmer module designs. 23-Apr-2005 Ampere Interrupting Capacity Page 5 of 11 Pages

6 5.0 BEWARE OF SIMPLISTIC FAULT CURRENT CALCULATIONS By Keith Lane, SASCO Specifying and installing underrated equipment can undermine your power distribution design Fault current calculations are a critical piece of the electrical design/engineering puzzle for electrical distribution systems in commercial and industrial installations. A fault current calculation determines the maximum available current that will be available at a given node, or location, in the system. Once the fault currents have been calculated, you can then select overcurrent protection equipment, breakers, and fuses with a fault current rating equal to or greater than those values (NEC 110.9). If a breaker or fuse isn't rated to handle the maximum available fault current it might see, it may not operate properly and its internal parts could fuse together or the device may blow up under the destructive stresses of a fault condition, which can cause serious injury and or property damage. Fault current calculations are based on Ohm's Law (V=I R). To determine the maximum current available at any given point in a distribution system, the equation is rearranged to solve for current (I=V R). In a short circuit condition, resistance (R) gets very small and is essentially based on the total resistance in the electrical distribution system, from the derived source of power to the point of the fault. As noted above, basic point-to-point fault current calculations are derived using Ohm's Law. System characteristics like voltage, conductor length, conductor constants, available short circuit values at the beginning of the circuit, and transformer percentage impedance are used to find the fault current at various locations within the system. AIC calculations for this roof-mounted distribution panel must include motor contribution factor. 23-Apr-2005 Ampere Interrupting Capacity Page 6 of 11 Pages

7 These calculations become more complicated when you consider that the resistance (R) should actually be replaced with an impedance value (Z). Impedance is calculated using the formula Z= (R2+X2). For instance, instead of using the percentage impedance for transformers, percent Z, percent X, and percent R are used and converted to X and R values on a per unit basis. And instead of using a defined constant for conductors, the impedance for conductors within the electrical system are also broken down into X and R components of the impedance. The ratio of reactance to resistance, or X/R, determines the peak asymmetric fault current. The total asymmetric current is a measure of the total DC component and the symmetrical component. The DC component causes the short circuit current to be asymmetrical. The asymmetrical component decays with time and will cause the first cycle of a fault current to be larger in magnitude than the steady-state fault current. In addition, the decay of the DC component depends on the X/R ratio of the circuit between the source and the fault. If the fault comprises all reactive components, then the resistance value of the X/R ratio is zero and the DC component will never decay. If the reactive component of the impedance is zero, then the DC component decays immediately. In the real world, impedance is neither all resistive nor all reactive. It's a combination of both. As you can see, calculating asymmetric current can be very difficult. Accurate calculations require you to know the rate of change of all reactive components in the system. However, multipliers, based on the actual calculated X/R ratios, have been developed to somewhat simplify this process. They're used in conjunction with the symmetric fault current calculation to provide an asymmetric fault current, which includes the DC offset component. The issue becomes more confusing based on the fact that all low-voltage protective devices are tested at predetermined X/R ratios. If the calculated X/R ratio at any given point in the electrical distribution system exceeds the tested X/R ratio of the overcurrent protective device, then you must derate the effective rating of the gear. All low-voltage protective devices are tested at predetermined X/R ratios. This can be a very critical issue if your fault current calculation doesn't include the X/R ratio or the peak asymmetric fault current. A fault condition with a high reactive component can potentially require a de-rating of the gear to below the symmetric fault current value 23-Apr-2005 Ampere Interrupting Capacity Page 7 of 11 Pages

8 calculated at the gear. Replacing installed gear that has been undersized because these asymmetric calculations haven't been provided could certainly ruin your day. In addition, induction motors can contribute short circuit current back into the system during a fault condition. The motor can act as a generator and contribute back EMF from the inertia of the load and the rotor driving the motor after the fault occurs. Because the flux is produced from the induction from the stator and not the field windings, the motor contribution is quickly reduced and will only last for a few cycles. The total motor contribution depends on many factors, including motor horsepower, voltage, the reactance of the systems at the point of the fault, and the reactance of the motor. The following example illustrates a potential problem of installing underrated electrical equipment if X/R ratios and motor contribution aren't considered in the initial design. Example design criteria: * Utility transformer rating: 2,500kVA * Utility transformer % impedance: 4.775% * Service conductors: 10 sets of 600 MCM copper * Available fault current at utility transformer secondary: 63,000A * X/R ratio at the utility transformer secondary: 11 * Motor contribution: 400 hp * Ampacity of service conductors: 4,000A * Service gear tested X/R ratio: 4.9 A fault current of 62,321A is calculated at the switchgear. This value is based on the 2,500kVA utility transformer with 4.775% impedance and minimal impedance from the service conductors (11 feet of 10 sets of 600 MCM copper). The simple form of this calculation, based on infinite bus theory, is indicated below: 2,500kVA ( 3 480V) =62,975, or 63,000AIC at the utility transformer secondary However, the AIC is further reduced at the service to 62,321A, based on the impedance of the service conductors. This assumes no contribution from the motors in the system or from the asymmetric component. 23-Apr-2005 Ampere Interrupting Capacity Page 8 of 11 Pages

9 If the switchgear is rated based on this information only (i.e. no X/R or motor contribution), the switchgear could have easily been specified as 65,000AIC. In this example, we're looking at a total contribution of 400 hp for the motor and an X/R ratio of 11 at the secondary side of the utility transformer. The X/R ratio at the switchgear is calculated at based on the X/R ratio at the service transformer provided by the utility and the contribution of resistance from the service conductors and the reactance from the motors in the system. The electrical service equipment has been tested and rated at an X/R ratio of 4.9. The calculation of this X/R value can be performed using sophisticated software, electronic spreadsheets, or by long perunit handwritten calculations. However, this work is beyond the scope of this article. This 4,000A service switchgear requires AIC bracing and overcurrent protection in excess of 65,000AIC. The motor adds a total of about 2,406A of fault current over the first half cycle. The motor contribution is based on the characteristics of the individual motors, but can be estimated by taking the total horsepower contribution, multiplying it by 5, and then converting this number to amps. The high X/R ratio requires the de-rating of the gear by a factor of The is a multiplier factor equal to the asymmetric current at the calculated X/R ratio divided by the asymmetric current at the tested X/R ratio. In addition, the following formula can be used to calculate the multiplier factor based on the calculated X/R ratio and the test X/R ratio for a given overcurrent protection device: In this case, the total fault current available, including the motor contribution, would be 64,727A. 23-Apr-2005 Ampere Interrupting Capacity Page 9 of 11 Pages

10 62,321A (symmetric fault current)+2,406a (motor contribution)=64,727a The gear, based on asymmetric de-rating, would be rated at 57,068A. 65,000A (gear rating) (X/R derating factor)=57,068a Assuming that the simplistic form of fault current calculations was initially used to size and install the main switchboard, the switchgear would have a fault current rating lower than the maximum available short circuit current and would therefore have to be replaced. More complicated issues can arise if closed transition paralleling (utility and generator) gear, parallel redundant UPS systems with closed transition bypass, or high-impedance grounding systems are used. In many cases you should rely on a qualified professional and the use of advanced software to ensure that electrical gear is properly rated to provide protection for personnel and property. Keith Lane is a registered P.E., RCDD/NTS specialist, LC, LEED A.P. and serves as vice president engineering at SASCO Design/Build Electrical Contractors and Consultants in Woodinville, Wash. Keith Lane's article was originally published in EC&M magazine Sep Apr-2005 Ampere Interrupting Capacity Page 10 of 11 Pages

11 Strand Lighting Offices Strand Lighting Italia srl Strand Lighting GbmH Via Delle Gardenie 33 Kurfürstendamm Pomezia-Roma, Italy Berlin Tel: Tel: Fax: Fax: Strand Lighting Strand Lighting Asia Novinsky Boulevard 20A, 20/F Delta House, Buildings On Yiu Street Moscow, Russia Shatin, N.T. Hong Kong Tel: Tel: (852) Fax Fax: (852) Strand Lighting Inc. Strand Lighting (Canada) Inc Darin Way, Eclairages Strand (Canada) Inc. Cypress, CA 90630, USA 2430 Lucknow Drive #15 Tel: Mississauga, Ontario Fax: L5S 1V3, Canada Tel: Strand Lighting Limited Fax: Unit 3 Hammersmith Studios, Yeldham Road, Hammersmith, London W6 8JF United Kingdom Tel: +44 (0) Fax: +44 (0) Apr-2005 Ampere Interrupting Capacity Page 11 of 11 Pages