
99
5
Maintenance inspections
5-2 Periodic inspections
Table 5-7 Inspection item and recommended countermeasure
Inspection item
Countermeasure
1. Measure insulation
resistance with a
500V DC megger
s)NSULATIONRESISTANCEISBELOW-
:
: Replace
the MCCB.
s)NSULATIONRESISTANCEISHIGHERTHAN-
:
but
less than 5M
:
: Conduct a dielectric strength
test (double the rated voltage for 1 minute).
The MCCB can be used temporarily as long
as test results indicate the voltage withstand
level is within specified limits, but replace the
MCCB as soon as possible.
s)NSULATIONRESISTANCEOFATLEAST-
:
: The
MCCB can be reused.
2. Conduct periodic
inspections
Take the recommended countermeasure
described in Table 5-5 if there is a problem.
5-2-3 Inspection following overcurrent tripping
When an MCCB trips due to overcurrent, visually inspect it
to see the extent of damage. Take action as prescribed in
Table 5-6 to prevent malfunction or failure if the MCCB is to be
used again.
Inspect the items in Table 5-7 and take the recommended
action if the extent of MCCB damage due to overcurrent
tripping is not clear or is as listed in Table 5-6.
Table 5-6 Extent of damage and recommended countermeasure
Extent of MCCB damage
Countermeasure
1. The area around the exhaust gas outlet
is clean and no other foreign matter is
visible.
The MCCB can be used
again.
2. Black soot is visible in the area around
the exhaust gas outlet.
See Table 5-7.
3. The exhaust gas outlet is noticeably
dirty and soot is clearly visible on the
handle.
Replace the MCCB.
Table 5-5 Inspection procedures
Inspection item
Procedure
Countermeasure
1. Presence or
absence of
dust or other
contaminates
Inspect the surface of the MCCB and especially around the
terminals on the power supply side to make sure no dust or oil
has accumulated.
Check to make sure that dust or other foreign matter has not
bridged the gap provided to increase creepage distance.
Vacuum up any dust, and wipe the area down with a dry, lint-
free cloth.
2. Exhaust gas
outlet
Inspect the exhaust gas outlet for foreign matter like carbon or
metal particles adhering near the outlet that may be evidence
of overcurrent tripping.
Replace the MCCB if soot or metal particles are found.
See items 6 and 7 in this table to decide whether to replace
the MCCB when only a minute amount of soot or metal
particles is found.
3. Discolored
terminal
Inspect the terminal for evidence of an abnormal temperature
rise or advanced damage due to corrosive gases like
hydrogen sulfide gas.
Some discoloration of silver plating is not a problem. Replace
the MCCB in case of advanced stages of discoloration or
damage to the insulation due to an abnormal temperature rise.
4. Loose terminal
screw
Check for loose terminal screws or cable tightening screws.
Use standard tools for tightening.
Confirm the normal tightening torque for the screws and
materials in advance, and never overtighten or undertighten
screws.
5. Switching
Intentionally switch several times to prevent excessive wear
due to grease hardening, and use the sliding action of
contacts to stabilize contact resistance with a normally closed
MCCB.
Repair MCCBs that do not switch smoothly.
6. Insulation
resistance
Use a 500V DC megger to measure insulation resistance
between the main poles as well as between live parts and
ground. Remove all conductors prior to taking measurements.
Replace the MCCB if the megger reading is below 5M
:
.
7. Temperature
rise
Check the following items through load current.
(1) The temperature of the molded case should not exceed
70°C.
(2) There should be no smoke or abnormal odors.
Replace the MCCB.