25-09-2012, 12:52 PM
Protective Devices Maintenance as it Applies to the Arc/Flash Hazard
[attachment=34542]
Introduction
FKey component of the Flash Hazard Analysis:
– Protective device clearing time.
uPrimarily circuit breakers and relays.
FFuses do not have operating mechanisms
FPrimary focus is maintenance issues
FMolded case and low-voltage power circuit breakers:
– Will generally clear a fault condition in 3 to 8 cycles.
– To be conservative a clearing time of 8 cycles should be
used.
F Older medium-voltage circuit breakers:
– Will clear a fault in around 8 cycles
– Newer ones clear in 3 to 5 cycles.
Molded-Case Circuit
Breakers
FMaintenance on molded-case circuit breakers is
limited to:
– Proper mechanical mounting
– Electrical connections
– Periodic manual operation
F Lighting, appliance, and power panel circuit
breakers
– Have riveted frames
– Are not designed to be opened
Molded-Case Circuit
Breakers
F All other MCCB’s that are UL approved:
– Factory-sealed to prevent access to calibrated elements.
F An unbroken seal indicates:
– Mechanism has not been tampered with
– Should function as specified by UL.
F A broken seal voids the UL listing and the
manufacturers’ warranty of the device:
– Integrity of the device would be questionable.
Low-Voltage Power Circuit
Breakers
F If the breaker remains open or closed for a long
period of time:
– Open and close the breaker several times
– Exercise under load conditions (hazard-remote operation)
F Environmental conditions play a major role.
FMore frequent inspections and maintenance may be
required if:
– Severe load conditions exist
– Inspection reveals heavy accumulations of dirt, moisture,
or other foreign matter
Medium-Voltage Power
Circuit Breakers
FMost requirements the same as low-voltage
power circuit breakers.
FBreakers should be removed from service and
inspected at least once per year.
FAlways follow the manufacturer’s
instructions.
Protective Relays
FRelays monitor complex power circuit
conditions, such as:
– Current and voltage magnitudes
– Phase angle relationships
– Direction of power flow
– Frequency
FWhen a short circuit (or fault) is detected:
– Relay responds and closes its contacts
– The abnormal portion of the circuit is
deenergized via the circuit breaker
[attachment=34542]
Introduction
FKey component of the Flash Hazard Analysis:
– Protective device clearing time.
uPrimarily circuit breakers and relays.
FFuses do not have operating mechanisms
FPrimary focus is maintenance issues
FMolded case and low-voltage power circuit breakers:
– Will generally clear a fault condition in 3 to 8 cycles.
– To be conservative a clearing time of 8 cycles should be
used.
F Older medium-voltage circuit breakers:
– Will clear a fault in around 8 cycles
– Newer ones clear in 3 to 5 cycles.
Molded-Case Circuit
Breakers
FMaintenance on molded-case circuit breakers is
limited to:
– Proper mechanical mounting
– Electrical connections
– Periodic manual operation
F Lighting, appliance, and power panel circuit
breakers
– Have riveted frames
– Are not designed to be opened
Molded-Case Circuit
Breakers
F All other MCCB’s that are UL approved:
– Factory-sealed to prevent access to calibrated elements.
F An unbroken seal indicates:
– Mechanism has not been tampered with
– Should function as specified by UL.
F A broken seal voids the UL listing and the
manufacturers’ warranty of the device:
– Integrity of the device would be questionable.
Low-Voltage Power Circuit
Breakers
F If the breaker remains open or closed for a long
period of time:
– Open and close the breaker several times
– Exercise under load conditions (hazard-remote operation)
F Environmental conditions play a major role.
FMore frequent inspections and maintenance may be
required if:
– Severe load conditions exist
– Inspection reveals heavy accumulations of dirt, moisture,
or other foreign matter
Medium-Voltage Power
Circuit Breakers
FMost requirements the same as low-voltage
power circuit breakers.
FBreakers should be removed from service and
inspected at least once per year.
FAlways follow the manufacturer’s
instructions.
Protective Relays
FRelays monitor complex power circuit
conditions, such as:
– Current and voltage magnitudes
– Phase angle relationships
– Direction of power flow
– Frequency
FWhen a short circuit (or fault) is detected:
– Relay responds and closes its contacts
– The abnormal portion of the circuit is
deenergized via the circuit breaker