Puzzled by choosing circuit protection devices? Follow these guidelines and your electrical system will fall into place.

Is it sufficient to select protective devices based entirely on their ability to carry a designated load and interrupt fault current in their part of the distribution system? No. An accurately engineered electrical distribution system will allow only the protective device closest to the fault to open, leaving the rest of the system undisturbed. Selectivity in overcurrent protection permits isolating a faulted circuit from the remainder of the electrical system, thereby eliminating unnecessary power outages.

When the main breaker trips in the middle of an urgent production run, brings your equipment to a sudden halt, and ruins an entire batch, it may be time to look into circuit protection devices. You may think an overload on a single branch circuit can't trip the main breaker, but it can. Your ability to stop it from happening depends on what you know about selectivity and circuit protection coordination and how you implement that knowledge.

Perform a selective coordination study.

There are two methods for performing a selective coordination study (SCS). The first is the use of time current curve overlays using manufacturers' published data, which are hand-plotted on log-log paper. The second method uses computer-based programs that allow the designer to electronically select data provided by the manufacturers. An SCS has four steps.

Step 1: Data collection. Gather data on the available fault current, the equipment rating and overcurrent device settings for the utility service and the distribution system. Include data for each service supply where multiple feeders serve the system under study. Ensure you know the manufacturer and model number (or catalog number) for each piece of equipment. Whether you use a spreadsheet, computerized maintenance management system (CMMS), enterprise asset management system (EAM), or selectivity software to store the information, you want it to hold all the pertinent information to allow follow-up action. Where a standby engine generator supplies the system, collect data for the available fault current and reactances of the generator and perform the study in both the normal and emergency conditions.

Step 2: Prepare a one-line diagram. Even a small impedance will limit short-circuit current magnitudes, so your one-line diagram must represent the entire system and show the following information:

  • Equipment: power, voltage, and current ratings.

  • Utility and generator sources: the available short-circuit current, protective relays, current-limiting settings, and any impedance or reactance data.

  • Transformers: kVA rating, liquid or dry type, primary and secondary connections (delta or wye), impedance, damage curves, and primary and secondary voltages.

  • Conductors: physical sizes, types, number of conductors per phase, insulation, lengths, and the material of the wireway (magnetic or nonmagnetic, aluminum or steel) and busway.

  • Motors: full load current, horsepower, voltage, and rpm. Note the reactance of all major motors — typically those 50 hp and above. If a motor has a variable frequency drive (VFD) associated with it, note the drive manufacturer and model.

  • Circuit breaker and fuses: ratings, characteristics, and ranges of adjustment for all overcurrent protective devices.

Step 3: Perform short-circuit calculations. In most cases, a 3-phase short circuit is the greatest short-circuit current a commercial or industrial electrical distribution system will encounter. So calculating a straightforward, 3-phase short circuit is adequate for determining the proper selection and setting of short circuit- and overcurrent-protective devices. The maximum short-circuit current will flow through a circuit breaker or fuse when the fault occurs at its load terminals. Thus, you must determine the level of short-circuit current at each location in the system.

Using the one-line drawing you created in Step 2, calculate the reactance and resistance of each circuit element in the system. The reactances and resistances are all line-neutral values using one phase of a 3-phase circuit. If you know the values for each generator, transformer, or motor, use them instead of calculated values or those you may find in a reference chart.

The impedance data should include the following:

  • Reactance and resistance of all bus and cable on systems operating at 600V and below.

  • Reactance only of generators, transformers, motors, and high-voltage cables. What about medium-voltage cables? Typically, you would include them — but not always. Sometimes we can ignore medium-voltage cable impedance, because it has little effect on calculations in the low-voltage system operating at 600V and below.

Determine the reactance and resistance of each circuit section by considering bus and cable length. For parallel conductors per phase, divide milliohm values of one conductor by the number of identical conductors in that section. After you've determined the short-circuit currents produced by the various sources throughout the system, you can calculate the short-circuit values at the locations you selected for study.

You can calculate the 3-phase short-circuit current at each location using one of the following four methods:

  • Ohmic method

  • Per-unit method

  • Computer software method

  • Point-to-point method

The ohmic method is useful for very simple electrical systems. It depends on calculated ohmic values to determine the short-circuit current at each designated location. However, the ohmic method doesn't generally apply to industrial or commercial systems because it's difficult to convert ohms from one voltage base to another without error. In addition, the small numbers used in the calculation process make accurate calculations tricky.

The per-unit method is useful in more complex designs, particularly when you must consider several voltage levels. It is more representative than the ohmic method when performing a conventional electrical circuit analysis. It establishes base, or reference, values for volts, current, kVA, and ohms and then refers the actual parameters of the system to these bases in equations to establish per unit values for each. Applying these values in unique equations provides the calculated short-circuit values.

The computer software method is the most popular because of its speed, ease of use, and ability to run multiple system design scenarios. Software calculates the short-circuit current at each selected location based on the point-to-point or traditional method, the per-unit or ANSI method, and — more recently — the IEC method.

The point-to-point method, sometimes referred to as the direct method, is a progressive analysis of the electrical distribution system. Starting at the source, you analyze each section to determine the short-circuit current at each designated location down to the end of the various circuits. Separately consider each point of fault.

Step 4: Device selection/settings. To localize the disturbance created by a fault, the protective devices should be selective in operation — so the one nearest the fault on its power source side will have the first chance to operate. If the device fails to function as required, the next device “upstream” must respond by opening the circuit. After calculating the short-circuit current at each selected location, select the characteristics of protective devices to gain this selectivity. You must select the fault-current clearing devices, such as fuses and instantaneous trips, or adjustable setting devices, to operate on the minimum current that will permit them to distinguish between fault current and normal load-current peaks. These must function in the minimum possible time and still be selective with other devices in series with them. Set the adjustable devices in the field to achieve the desired coordination.

To demonstrate the design concepts we've just presented, let's take a look at a practical example. We'll use the one-line in Fig. 1 on page 23 and Fig. 2 on page 26 (of the original article) for this analysis. The information you see is the result of a short-circuit study. The protective devices have adequate interrupt ratings. The one-line is a simple radial system and will involve two time-current curves.

The time-current curves include the primary fuse PD-0001 for the service transformer, the service transfer damage curve XF Service, service cable damage curves CBL-0001, time-current curve for the main circuit breaker PD-MCB, uninterruptible power supply input circuit breaker PD-UPS IN and mechanical equipment circuit breaker PD-HVAC. You have proper device coordination when you have no overlapping of curves (Fig. 1). Overlap of curves prevents coordination between PD-MCB and PD-UPS IN with the settings provided (Fig. 2). To better illustrate the limiting effect of conductors, note the calculated short-circuit current MSB in Fig. 2 is significantly less than it is in Fig. 1. This is because the current carrying capacity of cable CBL-0001 is less than in the previous analysis. The smaller cable introduces an impedance that limits the available short-circuit current at MSB and to all devices “downstream” from the main switchgear.

The proper selection and setting of short circuit protective devices is an essential part of providing adequate protection to a distribution system. Coordination limits the potential for a power interruption by involving only the necessary devices to clear a localized fault. The end result is more reliable, lower-cost operation.

Rafter is the President of Power Engineering, Inc., in Lenexa, Kan.