Protection and Control - Section 14 Flashcards
What device on a boiler is an O/c comparable to?
A pressure relief valve;
Safety valve.
What four factors must be considered when selecting proper fuses and CBs?
- Voltage rating;
- Continuous current rating;
- Interrupting rating;
- Speed of response.
What is the main difference between short circuit currents and overload currents?
Short circuit currents flow outside the normal current path;
Overload currents simply produce excess heat in the normal current path.
When is it permissible to round up the size of an overcurrent device using Table 13?
When protecting conductors rated under 600 amps, other than a few exceptions with smaller sizes.
What does interrupting capacity mean?
The maximum short circuit current that an overcurrent device can safely interrupt at rated voltage.
Does any circuit breaker have a minimum amount of time which it needs to respond to a fault?
Yes, due to the inertia of the moving parts.
What does current limiting mean?
Device must reduce heavy fault currents to less than that which would flow without the device.
Why is the quartz sand used as a filler in the current limiting fuse?
To quench the arcs and fuse to form an insulating barrier, helping to reduce the current to 0.
How fast can a modern fuse interrupt a fault current?
Within a few thousandths of a second.
Are all fuses useable in similar AC or DC systems?
No, generally with DC the same fuse can interrupt a lot less.
Can a class HRCI - T 300 V fuse be used to protect a 347/600 V wye system?
No, the phase voltage is more than 300 V.
Could a class cc fuse be installed in a standard fuse holder?
Yes, but the interrupting rating would have to be derated for 10000 A.
When does selectivity occur in coordination of a system?
When the time current curves do not cross one another.
Why must all fuses or CBs in a given panel be capable of interrupting the maximum fault current available at the panel?
So that every branch circuit is adequately protected.
When is the only time permissible to install a breaker of lesser IR rating than the available fault current?
In a series rated combination;
Both breakers must trip simultanaeously, which provides a cushioning effect for the branch circuit breaker.
