The most dangerous property of a capacitor is its very function: storing energy. You disconnect the source and feel reassured — yet the full charge waits inside the cylinder for whoever touches it. This page covers the rule that protects the lives of those working on capacitors.
Why Does a Capacitor Remain Dangerous After Disconnection?
A capacitor's function is to store electrical energy and retain it — so merely disconnecting the source does not mean the electricity is gone. The stored charge remains on its plates at a voltage that may equal the network voltage at the moment of disconnection (or its peak), and discharging it through a human body or through accidental contact between its terminals is a harmful discharge that can be lethal or cause damage.
Discharge Methods
- Internal means: many capacitors are manufactured with internal discharge resistors that automatically drain the charge within a specified time after disconnection.
- External means: if a capacitor lacks an internal discharge means, it must be provided with an external discharge device — this is the first safety precaution.
- Manual discharge and earthing: after the discharge time has elapsed, verify the voltage, then discharge the terminals and earth them with properly rated equipment before any contact — never rely on internal resistors alone.
The Practical Discharge Standard
The practical standard is that a capacitor's voltage should drop to below 50 volts within about 60 seconds (one minute) of disconnection. This period may extend to five minutes if the voltage exceeds 600 volts.
Why is 600 volts the threshold? Because it represents roughly the upper boundary of low voltage; anything above it falls into the medium-voltage category with its higher hazards.
Additional Precautions
- Always earth the metal enclosure to prevent any charge buildup on the capacitor's frame.
- For voltages above 600 and 1000 volts: adequate fences, walls, barriers, and clearances, given the severity of discharge at these levels.
- Link these rules to the general substation isolation procedures for maintenance: permit, disconnection, verification, discharge, earthing — then work.
Sample answer: Because a capacitor's function is to store energy, disconnecting the source does not remove the charge stored on its plates, and it can cause a harmful or lethal discharge upon contact. The practical standard: the voltage must drop to below 50 volts within about 60 seconds of disconnection, extending to five minutes if the voltage exceeds 600 volts — 600 volts is taken as the threshold because it represents roughly the upper limit of low voltage. After this time has elapsed, the voltage is verified, manually discharged, and earthed before work begins.
Blindly trusting internal discharge resistors. The resistor may be open-circuited without your knowledge — verification with a measuring instrument followed by manual discharge and earthing is a step that cannot be skipped regardless of the internal equipment provided.
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