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Determine if there is an exposure to a capacitor-stored energy hazard

Working Safely with �Hazardous Capacitors

ESM Section 15

11/03/2021

Field Guide 16

Revision 0.02

Capacitors >10 Joules and >100 Volts are considered hazardous at Berkeley Lab.

Capacitor Hazards and Thresholds

Threshold	Shock Hazard
0.25 J and 400 V	Reflex injury
10 J and 100 V	Internal burn injuries to nervous system Heart fibrillation

Threshold	Short-Circuit Hazard
100 J	Burns to skin
1 kJ	Burns + mechanical deformation of metal
10 kJ	Arc blast

Threshold	Arc Flash Hazard
100 J	Hearing damage
40-120* kJ	Arc-flash hazard
122 kJ	Lung collapse

Performing a Shock Risk Assessment

An exposure exists if any conductor or circuit part could potentially remain energized with hazardous stored energy and is not suitably guarded, enclosed or insulated.
All circuit parts that are electrically connected to a capacitor or capacitor bank may present this hazard, even if in a separate enclosure.

Determine the shock hazard using both the voltage and stored energy of the capacitor

Note that a high voltage capacitor is not a shock hazard below the energy threshold of 10 Joules, but can cause a reflex injury.
Use the DC shock approach boundaries (for capacitors above 100 V and 10 J) and PPE ratings.

Determine the total capacitor-stored energy of the system

If the equipment is not labeled, capacitor stored energy and wait time calculations should be performed by an engineering professional proficient with this type of work such as a PE or pulse power engineer. Contact your ESO for help.

*Note that the arc-flash threshold may be lower for a capacitor in a box, and there may be an additional contribution to the arc-flash energy from an AC or DC source connected to the capacitor bank.

OTHER HAZARDS

Subjecting a capacitor to high currents, such as a short circuit condition, may cause violent rupture of the capacitor enclosure
Rupture of a capacitor can create a fire hazard from ignition of the dielectric fluids. Dielectric fluids may release toxic gases when decomposed by fire or the heat of an electric arc
Long, high-voltage coaxial cables can have significant capacitance, see manufacturer’s information

See Electrical Safety Manual Rev2 Section 15 for detailed Capacitor hazard calculation instructions.

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Test and Grounding Methods

Determine the required test and grounding method.

Develop a Complex LOTO Procedure or Permit

Hazardous capacitor-stored energy is considered an additional energy source. Therefore, you need an approved complex LOTO procedure or permit for any mode 0/1 work with such equipment.

The complex LOTO procedure or permit should include:

All required steps to place the equipment into an electrically-safe work condition
The amount of stored energy available
How long to wait after de-energizing before opening the enclosure
How to test for zero absence of energy
What to do if there is still energy present

Label the Equipment

Label the equipment with a stored energy hazard warning label, so that qualified persons are made aware of the potential for stored energy and the need for a written discharge procedure. Include the stored energy in Joules, the voltage, and required wait time.

O. Sowle

August 31, 2021

	Test Method			Grounding Method
	0 - 100V	100 - 1kV	>1kV	0 - 100V	100 - 1kV	>1kV
< 0.25 J	no hazard	no hazard	no hazard	no hazard	no hazard	no hazard
0.25 - 100J	no hazard	Digital Volt Meter	DVM with HV adapter	no hazard	Hard ground ok	Hard ground ok
100 - 1kJ	Digital Volt Meter	Digital Volt Meter	HV Utility Grade DC Voltmeter	Hard ground ok	Hard ground ok	Hard ground ok
1k - 100kJ	Digital Volt Meter	Digital Volt Meter	HV Utility Grade DC Voltmeter	Soft ground then hard ground	Soft ground then hard ground	Soft ground then hard ground
>100kJ	Engineered Method	Engineered Method	Engineered Method	Remote grounding only	Remote grounding only	Remote grounding only

Hard grounding: Use an insulating stick with a grounding tip to discharge a capacitor between its two terminals and then to ground.

Soft grounding: Use an insulating stick with a mounted resistor or a Hi-Z point to perform a controlled discharge of the capacitor first. Then, after the prescribed wait time, apply hard grounding.

Wait time: If capacitors are equipped with bleed resistors, or if using a soft grounding system, determine the required discharge wait time, where applicable. After the required discharge time, apply a hard ground stick or hard grounding system.