Assuring dependability: field testing MCCBs

Effective maintenance of molded-case circuit breakers demands careful attention to minor details combined with a thorough testing program.Basic maintenance procedures plus proper electrical tests are essential to maximum reliability of molded-case circuit breakers (MCCBs). Maintenance and testing of these breakers are comparatively simple but sometimes thought to be difficult or complex. This misunderstanding

Effective maintenance of molded-case circuit breakers demands careful attention to minor details combined with a thorough testing program.

Basic maintenance procedures plus proper electrical tests are essential to maximum reliability of molded-case circuit breakers (MCCBs). Maintenance and testing of these breakers are comparatively simple but sometimes thought to be difficult or complex. This misunderstanding is a result of the many and varied styles and configurations of MCCBs on the market.

In general, the same tests are performed on MCCBs as on air-frame circuit breakers. The only reason the procedures for conducting these tests may be different is because of the physical configurations of the breaker types.

Maintenance procedures

Generally, the maintenance procedures and tests regularly done on MCCBs include visual inspection; lubrication; cleaning; insulation resistance tests; contact resistance tests; and overcurrent tests.

Visual inspection, lubrication, and cleaning of an MCCB are some of the simplest tests and therefore are sometimes overlooked. Yet, they are a vital part of breaker maintenance. These basic tests don't take much time to do, yet they can point out and help avoid catastrophic problems.

Insulation resistance, contact resistance, and overcurrent tests require specialized testing equipment for proper testing. Depending on the circuit breaker types and sizes to be tested, this equipment can be either inexpensive or costly.

Visual inspection. When performing a visual inspection, look for signs of overheating, excessive arcing, bent linkages, cracked insulation, tracking, etc. For a 3-phase breaker, this inspection is simplified because the condition of one phase can be compared with the other two phases.

For MCCBs, this inspection is sometimes difficult if not impossible. Some MCCBs come from the factory sealed. This means the only inspection that can be carried out is on the enclosure and cable terminations. If the MCCB is not sealed, the cover can be removed. This will allow inspection of current-carrying conductors and the mechanism. Again, there are some MCCBs that have arc chutes sealed in place. This makes the inspection of the moving and stationary contacts difficult.

Lubrication. The lubrication of circuit breakers is important and yet sometimes overlooked. If a breaker is not properly lubricated, it may not operate when necessary. Lack of lubrication could cause a reduction in the velocity of the moving contacts, thus disabling the breaker's ability to interrupt the fault currents.

If the MCCB is sealed, lubrication is impossible. Therefore, it should be exercised to distribute the contained lubrication within the breaker to the moving parts.

Cleaning. Cleaning of a circuit breaker goes hand-in-hand with lubrication. If the mechanism is not clean, the breaker may not operate as needed. Clean the breaker with a vacuum, and wipe it down with cotton rags. A vacuum is preferred over compressed air because the vacuum will draw dirt from the breaker. While the compressed air will blow most of the dirt off the breaker, some could be blown deeper into the unit. When the breaker is wiped down, give special attention to the insulation. Clean insulation reduces the possibility of tracking or a possible flashover.

Test techniques

Insulation resistance. The insulation systems in modern low-voltage circuit breakers are good. It is rare to hear an MCCB failed because of its insulation. Nonetheless, it is beneficial to test the insulating components.

To test a circuit breaker rated at 600V or below, an insulation-resistance tester that has an output of 1000V DC is a good choice. First, with the breaker closed, check the insulation between the phases. Next, test insulation between the line and load terminals with the CB open. Because the frame of an MCCB is not conductive, a phase-to-ground test need not be done.

Generally, the insulation resistance of an MCCB and air-frame circuit breaker should be greater than 50 megohms.

Contact resistance. There has always been a discrepancy between the manufacturer's suggested test procedures as compared with the actual field test. Typically, manufacturers and some testing specifications call for testing of contact resistance to be done with a DC test current equal to the circuit breaker rating. For manufacturers of circuit breakers, this test provides the most accurate information on the condition of the contacts. However, performing a contact resistance test of this type in the field is impractical. A DC current supply providing currents of this magnitude is large and expensive.

Therefore, for field testing, a digital low-resistance ohmmeter having the capability of delivering 10A of DC current and a resolution of 1 microhm should be used. Such an ohmmeter with this output capability and accuracy is commonly available. The physical size of such test devices is not large, and they can be transported without special equipment. Understandably, with the reduced capabilities of the smaller digital low-resistance ohmmeter, the results of the field test will not correlate with the manufacturer's method.

As a result, a good rule of thumb to follow is to investigate any breaker whenever a 50% deviation of resistance values appears between any pole.

Note: It is important that this test be performed before the overcurrent trip test. This is because most MCCBs utilize a thermal/magnetic trip device After an overcurrent trip test, the bimetallic strip, which performs the thermal trip, has absorbed significant energy and is at a higher than ambient temperature. This increase in temperature will cause errors in a contact resistance measurement. If the overcurrent test is performed before a contact resistance test, the breaker should be allowed to cool for 20 min before contact resistance is checked.

Overcurrent test. This test ensures that the series overloads operate the breaker within the specified tolerances. It is performed by injecting current into the circuit breaker to simulate the overload or fault condition.

For MCCBs, the typical overcurrent test calls for the trip unit to be tested for an overload and a catastrophic fault condition. The overload portion of the trip unit is tested by injecting current equal to 300% of the circuit breaker's rated continuous current. The instantaneous portion of the trip unit is tested by injecting short pulses of current (5 to 10 cycles long) below the instantaneous trip point and slowly increasing the amount of current until the CB trips.

When the overload trip test is conducted, the time from the application of current until the breaker trips is the critical data needed to ascertain proper operation. This data is readily available from The National Electrical Manufacturers Association (NEMA). NEMA Standard AB4-1991 provides the maximum trip times for circuit breakers and describes proper procedures for field testing of MCCBs.

When testing for the instantaneous trip point, the pulse-mode test is the preferred method. The test current pulses are set slightly below the instantaneous trip point, and pulsed current is increased until the breaker trips. The actual trip current is not usually critical in field testing. The primary concern is that the breaker will trip for an instantaneous fault. The actual trip current can vary as much as +40% to -30%.

For circuit breakers with nonadjustable instantaneous trips, tolerances apply to the manufacturer's published trip range, i.e.: +40% on high side, -30% on low side.

Low-voltage high-current tests

Because of the physical configuration of MCCBs, the actual connection between a low-voltage high-current supply and the breaker can be difficult. The primary problem is that high-current supplies utilize low-voltage output to deliver the appropriate high currents. This requires short, high-amperage capacity cables to reduce voltage drop. Because high-amperage capacity cables are difficult to handle, smaller cables are preferable. The problem with smaller cables is that the resistance of the cables increases as cable size decreases. The higher-resistance cables will in turn reduce the capabilities of the low-voltage high-current supplies.

Therefore, a compromise must be reached between the convenience of using smaller flexible cables and the capabilities of the low-voltage high-current supply.

There are a number of different connection "tricks" used in the industry to meet this compromise. One is to fabricate 12-in.-long, 1/2-in.-round copper rods to 10 ft long 4/0 welding cables. This connection scheme allows a technician to hold the copper rods (using all safety procedures) on the line and load terminals of the MCCB while the breaker is still mounted in the switchgear. A second technician operates the high-current test set. This provides the most convenient and effective way to connect a high-current test set to a breaker. However, the long leads (10 ft) and the additional resistance between the copper rod and the breaker add up to reduce the test set capabilities. This must be taken into consideration.

The second way to connect an MCCB to a high-current supply is to change the ends of the output leads. This is done by connecting a temporary cable splice to the end of a 4-ft length of 4/0 AWG welding cable. A temporary cable splice is a cable splice that fastens to the cable by means of screw terminals. Fabricate several pairs of copper rods approximately 5 in. long of various diameters. Suggested diameters are 1/8 in., 1/4 in., 1/2 in., 3/4 in., and 1 in. Depending on the size of the circuit breaker terminal, select the appropriate diameter and insert it into the temporary cable splice. This provides for easy connection changes between different size MCCBs. This assembly also addresses the voltage drop, thus allowing a high-current test set to operate properly.

John Shanks is Product Manager, AVO International, Dallas, Tex.

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