Jack Wulfmeyer, P.E.
In order to properly engineer a low voltage ( 600 volts or less) electrical distribution system, we must consider many parameters.
One of the most important parameters is a consideration of the short circuit protection as well as the overload protection. Too many times the protective device, whether it be a fuse or a circuit breaker, has just been selected on the ampacity of the circuit to be protected. That is, if you pick a wire capable of withstanding 100 amps, you pick a 100 amp fuse or circuit breaker. This may not be sufficient, since only one requirement is covered and there are several other important requirements that need to be considered.
You must also select a protective device so it will have the proper interrupting rating. That is, it must be able to open safely for any value of short circuit current that could occur in that particular part of the system. This is a completely different rating than the normal running ampere rating of the protective device. It is the maximum current that would flow if there were a bolted short in the circuit.
The other area, which is greatly ignored, and must be considered here, concerns protecting components: such as, switchgear, bus duct, motor control centers, motor starter (including their contacts and overload heaters), motors, wire, transformers and circuit breakers.
THE NATIONAL ELECTRIC CODE (NEC)
These areas of protection are covered in the 1996 National Electric Code. The seven articles in a separate NEC book will be discussed in more detail. The articles are 110-9, 110-10, 230-95, 230-65, 240-1, 430-52, and 450-3.
Section 110-9 - Interrupting Capacity. After reading the article you will find that it simply means any device used to protect a low voltage system should be capable of opening all fault currents up to the maximum current available at the terminal of the device. Many overcurrent devices, today, are used in circuits that are above their interrupting rating. This is a direct violation of paragraph 110-9 of the NEC. By using properly sized Current Limiting Fuses ahead of these devices, the current can usually be limited to a value lower than the interrupting capacity of the overcurrent devices.
Section 110-10 - Circuit Impedance and Other Characteristics. This paragraph states that the overcurrent protective devices, along with the total impedance, the component short-circuit withstand ratings, and other characteristics of the circuit to be protected shall be so selected and coordinated so that the circuit protective devices used to clear a fault will do so without the occurrence of extensive damage to the electrical components of the circuit. In order to do this we must select the overcurrent protective devices so that they will open fast enough to prevent damage to the electrical components on their load side.
Section 230-95 - Ground-Fault Protection of Equipment. This section means that 277/480 volt "wye" only connected services, 1000 amperes and larger, must have ground fault protection in addition to conventional overcurrent protection. The ground fault relay (or sensor) must be set to pick up ground faults which are 1200 amperes or more and actuate the main switch or circuit breaker to disconnect all ungrounded conductors of the faulted circuit.
Section 230-65 - Available Short Circuit Current. Sort of a repeat of 110-9, but, was put in to make it all the more important to have proper interrupting capacity. Service equipment shall be suitable for the short circuit current available at its supply terminals.
Section 240-1 - Reading this you will find that the real purpose of the overcurrent protective device is to open the circuit so that the overcurrent will not cause an excessive or dangerous temperature in the conductor, or conductor insulation. One must look further in the Code to find out how to apply this section. Fortunately, the Insulated Power Cable Engineers Association (I.P.C.E.A.) have published short circuit tables for conductors which tell how much current, for how long a time, it will take to damage a conductor. These published tables show; for example, that a #10 THW wire is damaged if it carries 4300 amps for one cycle, the normal opening time of a molded case circuit breaker.
Section 430-52 - Rating or Setting for Individual Motor Circuits. Gives the information on how to select the maximum, and we repeat, the MAXIMUM size overcurrent protection for individual motor circuits. Paragraph (c)(2) states that "Where maximum branch-circuit, short-circuit, and ground-fault protective device ratings are shown in the manufacturer's overload relay table for use with a motor controller or are otherwise marked on the equipment, they shall NOT be exceeded even if higher values are allowed as shown above." In many cases these fuses are either 3, 6, 9, or 10 ampere, and not the normal 15 amps as called out in other areas of the Code for small motors. This information on each starter has been generally ignored and it certainly cannot be ignored by consulting engineers, inspectors, or contractors installing the motors and motor starters. The manufacturer gives a 3 ampere fuse as the maximum for an overload heater rated approximately one amp. They mean a 3 ampere fuse is maximum, and not a 15 ampere fuse, as is used many times. Using a 15 ampere breaker would also be a violation.
Section 450-3 - This section is divided into three parts.
(a) Transformers over 600 volts, Nominal
(1)For primary and secondary protection with a transformer impedance of 6% or less, the primary fuse must not be larger than 300% of primary Full Load Amps (F.L.A.) and the secondary fuse must not be larger than 250% of secondary F.L.A.
(b) Transformers 600 volts, Nominal or less
(1)For primary protection only, the primary fuse must not be larger than 125% of primary F.L.A.
(2) For primary and secondary protection the primary feeder fuse must not be larger than 250% of primary F.L.A. if the secondary fuse is sized at 125% of secondary F.L.A.
(c) Potential (Voltage) Transformer
(1) These shall be protected with primary fuses when installed indoors or enclosed.
MOTOR STARTERS
Looking at some other parameters of protection we find in UL Bulletin 508 - for Motor Starters, they provide a test circuit that will produce 1,000 amperes in short circuit current on starters up to 1 H.P. at 300 V. or less, 5,000 amperes on starters 50 H.P. or less, 10,000 amperes on starters 50 H.P., to 200 H.P., and 18,000 amperes on starters 201 H.P. to 400 H.P., 30,000 amperes on starters 401 H.P. to 600 H.P., 42,000 amperes on starters 601 H.P. to 900 H.P., and 85,000 amperes on starters 901 H.P. to 1600 H.P. Some manufacturers will rate their own starters according to NEMA standards at a higher value, but, this cannot be assumed, and you must get data on the starter should you wish to apply it where fault currents may exceed the test fault ampere figure. The safest alternative would be to use Time Delay Class J current - limiting fuses, such as Edison Fuse's JDL series fuses, sized at 150% of motor F.L.A.
PROTECTION OF "EXAMPLE" SYSTEM (see the one-line diagram below)
To see how component protection really works in an actual system, we will look at a One-Line diagram for a typical low voltage electrical distribution system. We shall determine: (1) the switchgear bracing required, (2) the motor control center bracing required, (3) the starter protection required, (4) the motor protection, (5) the wire protection, (6) the circuit breaker protection, (7) the transformer protection, and (8) the bus duct bracing required. We will use the Edison Fuse fuses that provides the superior protection concept found in the international standard for low voltage switchgear and controlgear, I.E.C. 947, type 2 coordination. Page references are to the Edison Fuse Product Catalog.
SYSTEM MAIN SWITCHBOARD (1)
Looking at the One-Line Diagram, we first note that the available fault current F1 at the main switchgear is 120,000 amps rms symmetrical. We note that the interrupting rating of all fuses in the main switchgear is 200,000 sym. rms amperes. Next we need to look at the Fuse Let Thru Tables for an Edison Class L fuse LCU 2000 (page 13) and we find that the let-through current is 47,000 amperes (using 150,000 A rms short circuit). Now, this means that our main switchgear has to be braced to withstand at least 47,000 amperes, and the bracing then would be 50,000 amps, which would have to be specified. The main switchgear now meets Art. 110-9, 110-10, and 230-65 of the NEC.
MOTOR CONTROL CENTER-BUS BRACING (2)
The next area of component protection we look for is the motor control center. Note that the short circuit amperes available at the motor control center F2 is 84,507 amps rms symmetrical. Again, looking at the Fuse Let Thru Tables for Edison LCU 800 Class L fuses (page 13), using 100,000 A rms short circuit, we note that the LCU 800 fuse, with an interrupting capacity of 200,000 amperes, will allow an effective let-through current of 23,000 amps. This means that our motor control center bus must be braced to withstand at least 23,000 amps, rms symmetrical, and therefore, a standard 42,000, (some offer 65,000) ampere braced motor control center is satisfactory. The motor control center now meets Art. 110-9 and 110-10 of the NEC. If the 800 amp protective device for the motor control center had been a standard circuit breaker, the 84,507 amperes would have been fully available at the motor control center, and therefore, we would have to have it braced for the next higher rating of 100,000 amperes.
MOTOR CONTROL CENTER-STARTER PROTECTION (3) (4)
The next area of protection we are going to look at is probably one of the most important we will consider. This is the fault that can occur at the 10 H.P. motor, along with it's starter and overload relay. The One-Line Diagram shows F3 at 21,739 amps rms available at the motor starter. Looking at the fuse Let Thru Tables for Edison Class J Time-Delay (page 19) we find that the JDL 17 1/2 is not on the chart. We will use a JDL 30 which is the largest fuse that can fit in a 30 ampere disconnect and will give us an even higher value of let-through current, plus using 25,000 amps (instead of 21,739 amps) rms short circuit. We'll find even this oversized fuse limits the effective let-through to 1,440 rms amps. The JDL 17 1/2 fuse, with an interrupting capacity of 200,000 amperes, was sized and selected from the Edison Application Information (page A14) for a 10 H.P. motor at 460 volts. This Class J Time-Delay Fuse will give short circuit protection, and also backup overload protection for the motor and overload heater.
We should also select the size of the overload heaters, and to do this we would look in the starter manufacturer's table. The table we have selected is from Westinghouse Electric Corporation. We'll find that for a motor with 14 amps - full load - which is normal full load current for a 10 H.P. motor at 460 volts, we would select heater FH45. Note that at this table they also have the maximum fuse allowable and it is 45 amps, thus, we are well under that size since we've chosen the JDL 17 1/2 amp fuses.
Now, we have several things to consider to make sure that motor and starter are protected. The first is to be sure that the heaters are protected, and we noted that maximum fuse size allowed by the manufacturer is 45 amps, and we are below that because we are using a 17 1/2 amp. Unless the manufacturer has some short circuit ratings developed for the motor starter we should look to see if it has a UL label on it. If it does, we know that UL Bulletins 508 requires starters up to 50 H.P. to withstand the 5,000 amperes short circuit test. The Edison Class J Time-Delay Fuses lets through less then 1,440 rms amperes, therefore we know that the motor starter contacts and overload heaters will not be damaged because this is far less than what the starter was tested at.
WIRE PROTECTION (5)
The next area we should look at is our #12 THW wire. From the IPCEA table we'll find #12 THW wire* (* extrapolated) can withstand 2800 amperes for one cycle (1/60th of a second). Here again, the let-through of the JDL 17 1/2 fuse is less than 1,440 amps, and the fuse is clearing in less than 1/2 cycle, therefore, we've protected the wire. The motor circuits now meet Art. 110-9, 110-10, 240-1 and 430-52 of the NEC.
USE OF CIRCUIT BREAKER (4) (5)
We shall now consider other factors. If we had not used the Edison Class J Time-Delay fuse to protect the motor, but had used a circuit breaker in a combination starter we would have a different situation. Standard circuit breakers are not current limiting and, therefore, would let through the full effect of the 21,739 amperes. This would exceed the UL rating on the starter which is only 5,000 amperes, and therefore, we could expect serious damage to the contacts and overload heaters violating paragraphs 110-9, 110-10, 240-1, and 430-52 of the NEC.
Investigating this situation further, we'll remember that the wire tables for #12 THW wire* (* extrapolated) indicated a damage point of 2800 amps for one cycle. The circuit breaker, if it was used, would take approximately one cycle to open, and would allow the full 21,739 amperes short circuit current to flow. Therefore, the insulation and the copper wire would be severely damaged with this type of protection. A clear violation of paragraph 240-1 of the NEC.
One must remember, of course, that if we had used the circuit breaker, it must also have an interrupting capacity of at least 21,739 amperes or a standard 22,000 AIC, (NEC 110-9).
LIGHTING PANEL - CIRCUIT BREAKER PROTECTION (6)
Continuing on the One-Line Diagram we find that the 227-480 volt lighting panel P1 has a fault F4 of 52,632 rms symmetrical amperes short circuit current available. In order to determine the interrupting capacity of the circuit breakers in panel P1, we would check with the manufacturer. The use of an Edison Class J Time-Delay fuse, a JDL 200, may also allow you to use circuit breakers with an interrupting capacity of 14,000 amperes depending on the manufacturers test per UL 489. What interrupting capacity circuit breakers would be needed in the lighting panel if a current limiting fuse? Answer: 65,000 A.I.C.
TRANSFORMER PROTECTION (7)
Continuing down the One-Line Diagram we find that the next area to be protected will be the 75 KVA Dry Type Transformer - feeding lighting panel P2 at 120/208 volts. First thing we want to look at is the transformer and we find that a 75 KVA has a primary amperage of 90.3 amps at 480 volts. Taking a look at the NEC, Article 430-3(b) (2) recommendation for this application we select an Edison JDL 175 fuse to fit into the 200 amp switch. This will give us not only short circuit protection for the wire, but also short circuit protection and some degree of overload protection for the transformer. This meets Art. 110-9, 110-10 and 450-3 of the NEC. The secondary conductors may still need protection. Article 450-3 (b)(2) calls for the secondary fuse to be sized at 125% of FLA (209A) or 261.25A. We can choose to go lower and use a Edison JDL 200 fuse for the 200 ampere panel.
Investigating the transformer circuit further, we find that fault F5 available at panel P2 is 10,059 rms symmetrical amps. In order to figure out the effective let-through current at this point, from the Edison Class J Time Delay Fuse, we'll have to do some mathematics. Dividing 10,059 by 2.3 (which is the ratio between the 480 volt and 208 volt) we get 4,373 amps. Now looking at the Fuse Let Thru Tables (page 54), using a 5000 A rms short circuit, we'll find that the Edison JDL 200 fuse has an effective let-through current of 2,760 amps. This figure is on the primary of the transformer, and to get it to the secondary, or 208 volt side, we'll have to multiply it times 2.3. This gives us 6,348 rms amps as a let-through current. Since the 120/208 volt lighting panel breakers normally have an interrupting capacity of 10,000 amps, we may have met paragraph 110-9 of the NEC. One would have to check with the manufacturer to make sure their circuit breakers would meet paragraph 110-9 of the NEC and UL 489.
PLUG-IN BUS (8)
The next area to protect is the 600 amp bus duct. This 600 amp bus duct is protected by an Edison JDL 600 ampere fuse, with an interrupting capacity of 200,000 amps. The fault current F6 available on the bus duct is 114,286 rms symmetrical amps. Looking at the Fuse Let Thru Tables (page 19) for an Edison JDL 600 fuse, (using a fault of 150,000 A) we find that the effective let-through is 19,270 amperes. (This means that the bus duct must have a short circuit withstandability rating of at least 19,270 amperes). a standard (NEMA) 600 ampere bus duct has a symmetrical rms rating of 22,000 amps which is adequate for this let-through current. This will meet Art. 110-9, 110-10, and 240-1 of the NEC. Keeping in mind here if we had used a 600 amp circuit breaker to protect this bus duct, the bus way would have to withstand the full amount of an 114,286 ampere fault and therefore, would have to be braced for 150,000 amperes. The interrupting capacity of the circuit breaker would also have to be 150,000 amperes.
CONCLUSION:
In todays modern systems it is essential that all overcurrent protective devices are selected to adequately protect the circuit components and have an adequate interrupting rating. Modern current-limiting fuses are able to protect circuit components from high available short-circuit currents as well as overload currents and moderate fault currents. The Edison Fuse Product Catalog can be used to analyze an electrical system for component protection. An effective specification would call for all circuits over 600 A to use Edison LCU fuses (Class L) and all circuits 600 A or less to use Edison JDL fuses (Class J Time-Delay). Most all requirements for circuit protection, fault currents and component protection can be met with the Edison Fuse fuses which provide superior protection with basically no damage to components.
