FUSE AND CIRCUIT BREAKER MEASUREMENT PROCEDURES

StepExpected Result/Action
NOTE: Steps 4 and 5 check the condition (good or open) of the disconnect box
fuse(s).
4. Set disconnect switch to off. Using an insulated fuse puller, remove fuse(s)
from the disconnect switch.
Fuse(s) removed from disconnect switch in preparation for measurement.
Set up the VOM/DMM to measure continuity. If using a VOM, select the R x 1
range. Measure the resistance across the fuse as shown in Figure SP-7-5,
View A.
Fuse is good if VOM/DMM reads zero ohms. The DMM may also beep or buzz.
Fuse is bad if VOM/DMM reads an open or a measurable resistance.
Fuses can fail because o f poor contact in the disconnect switch. Check disconnect
switch for loose connections, discolored or hot clips, blades, etc.
NOTE: Steps 6 through 9 check the condition (good or bad)
of the input power or distribution panel circuit breaker(s).
6. At the power distribution panel, set the circuit breaker to off. If required,
remove the panel that covers the circuit breaker to expose the body of the
breaker and the wires connected to its terminals.
Circuit breaker is turned off.
Set up the VOM/DMM to measure AC voltage on a range that is higher than
the highest voltage expected. (Refer to the unit nameplate.) As shown in
Figure SP-7-5, View B, measure the voltage applied to the circuit breaker
input terminals: A to neutral or ground (single-pole breaker), A to B (2-pole
breaker), or A to B, B to C, and C to A (3-pole breaker).
Voltage is applied to the input of the circuit breakers, and it is within ±10% of the
power supply volts required for the unit as marked on the unit nameplate.
For example, our typical heating/cooling unit should have an input voltage that
ranges between 207 volts and 253 volts (±10% of 230 volts), measured across
the circuit breaker input terminals A and B.
Make sure that the breaker is closed by first setting it to the off position,
then to on. Measure the voltage at the circuit breaker output terminals A1 to
neutraf or ground (single-pole breaker), A1 to B1 (2-pole breaker), or A1 to
Bl, B1 to Cl, and Cl to A1 (3-pole breaker).
Voltage is measured at the output of the circuit breakers. If the voltage is
significantly lower than that measured at the input to the circuit breakers in step
7, visually inspect breakers for loose wires and terminals or signs of overheating.
If none are found, replace the circuit breaker.
If breaker shows signs of overheating or trips when voltage is applied to the
equipment, reset it, then check the current flowthrough the breaker per step 9.
Measure the current flow through the input power circuit breaker(s) as
follows:
Set up the AC clamp-on ammeter to measure AC current on a range that is
higher than the highest current expected. Check the ampere rating of the
breaker. It is usually stamped on the breaker lever or body. Refer to Figure
SP-7-5, View C. One wire at a time, measure the current flow in the wires
connected to the circuit breaker output terminals: A1 (single-pole breaker),
At, Bl (2-pole breaker), or A l, Bl, and Cl (3-pole breaker).
Compare the measured current with the breaker ampere rating. If the measured
current through the breaker is higher than the rating, an overload condition
exists in the breaker load circuit. If the breaker is tripping at a current below its
rating or not tripping at a higher current, replace it. Be sure the breaker is not
being tripped due to excessively high ambient temperature, such as might exist if
the box is in direct sunlight, in hot desert areas, or on roof tops. Make sure
HACR-rated circuit breakers are being used to power HVAC equipment.

pic1 89 FUSE AND CIRCUIT BREAKER MEASUREMENT PROCEDURES


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