A capacitor bank rated 480 volts is installed for power factor correction. What does the Code require regarding the charge stored in those capacitors?
- The residual voltage must be reduced to 50 volts or less within 1 minute after disconnection
- BThe residual voltage must be reduced to zero within 5 minutes after the bank is disconnected
Zero volts in five minutes is neither the threshold nor the time the Code states, and requiring absolute zero would be unverifiable in practice.
- CA grounding switch must be closed by the electrician before the enclosure door can be opened
A manual grounding switch is not permitted as the means of discharge, because it depends on a person doing something before the hazard is removed.
- DThe terminals must be short circuited with a jumper by the servicing electrician within 15 minutes
Shorting the terminals by hand is a field practice on high-voltage equipment, not the automatic discharge means the section requires.
Why A is correct
The rule sets a measurable performance requirement: 50 volts or less in one minute for units rated 1000 volts and below. Higher-rated capacitors get a longer allowance. The discharge means must operate automatically, which is why the section also prohibits relying on a manual switching operation to start the discharge.
What this question is testing
The item tests recall of a specific performance requirement expressed as a voltage and a time, and whether the candidate understands why the discharge must be automatic.
On the job
A disconnected capacitor bank can hold a lethal charge, and the person opening the door is often not the one who switched it off. The Code requires the discharge to be automatic for that reason. Field practice still calls for verifying with a meter and grounding the terminals before touching them, because discharge resistors fail open silently and nothing about the enclosure will tell you that it has happened.
Memory technique
Fifty volts in one minute, all by itself.
Exam tip
Capacitor questions almost always turn on stored energy: how much, how fast it drains, and whether draining is automatic.
Where to look it up
Look in the capacitors article at the section covering the drainage of stored charge.