Protection Devices Flashcards
What is meant by overcurrent?
An excess of current.
Caused by either short circuits or overloads
What do overcurrent devices protect against?
Short circuits
Overloads
Ground faults
Lightning surges.
What is the active function of a fuse?
To open a circuit under abnormal current conditions.
What is the most common type of fuse used?
Cartridge fuses.
Do most non-time delay fuses contain a thermal cut-out?
No.
The thermal cut out provides overload protection through a soldered connection to the fuse link.
What percentage of rated continuous current would a thermal cut out provide a time- delayed response?
Up to 500%.
At fault levels above 500%, the fuse link would open quickly.
What are the two basic types of circuit breakers?
Instantaneous (Magnetic) Thermal magnetic (time-limit).
What is the most common type of breaker?
Do these provide short circuit, or overload protection?
Thermal magnetic.
Both.
What does “trip free” mean?
It means that a breaker is “free to trip” even if mechanically held open.
Do most motors require overload protection?
Yes.
Due to a high inrush current on start the overcurrent device must be oversized and therefore only works to interrupt short circuit current.
What are the two main types of overload devices?
Thermal.
Magnetic flux.
What are the end blades of a fuse called?
Ferrules.
What kind of material are fuses generally filled with?
Arc quenching material,
Which table in the CEC is used to size overcurrent devices for a motor?
Table 29.
What are the two amperage ratings on an overcurrent device?
Continuous rating.
Interrupting rating.
Under short circuit conditions, how fast would a thermal magnetic breaker trip?
3-5 milliseconds.
What are the primary factors which determine fault current?
Circuit voltage.
Impedance of short circuit path.
kVA rating of supply transformer.
percent impedance of the supply transformer.
Can a fault current be higher than expected in a circuit with motors?
Yes.
Motors act as generators when decelerating and can increase the fault current.
What criteria must be observed to select a proper motor disconnect?
Manual action.
Lockable.
Break all ungrounded conductors @ locked rotor current.
Acceptable horsepower rating.
What are acceptable motor disconnects?
Fused/unfused switch.
MMS (manual motor starter/switch)
Circuit breaker
Plug (portable motors).
When are manual motor starters frequently used?
For small motor loads under 1/3 hP.
What is a motor magnetic?
An assembly consisting of a contactor and an overload relay.
When sizing an overload heater, what values must be considered?
Motor FLA (full load current), Motor service factor.
