You discharge it, walk away, and it comes back.
Article 370 is new, and it exists because electrical double layer capacitors do something ordinary capacitors do not: they rebuild a residual charge after being discharged, with no external circuit involved. Everything else in the article follows from that one physical fact.
370.1 covers the electrical safety requirements for safeguarding employees while working with exposed electrical double layer capacitors (EDLCs) that present an electrical hazard. Article 320 explicitly carves EDLCs, supercapacitors and hybrid supercapacitors out of the ordinary capacitor article and sends them here.
§ Thresholds and theoretical maximum power
370.2 does not invent its own numbers. Risk control measures are selected from the hierarchy of risk control where any of the hazard thresholds identified in 310.2 are exceeded — the common dc baseline.
The Informational Note to 370.2 gives the theoretical maximum power for EDLCs as V × V / (4 × R) — where V is the initial voltage and R the equivalent series resistance. Note the structural echo of the battery formula in Article 360: both divide by four, and both produce a power number that gets compared against the 1000 W contact thermal threshold.
370.3(B)(4) then ties PPE selection directly to that number:
| Hazard | Trigger | PPE |
|---|---|---|
| Contact thermal | Theoretical maximum power of the exposed part > 1000 W | Thermal PPE per 130.7(C)(7)(e) |
| Electric shock | Voltage > 100 V dc | Electric shock PPE per 130.7(C) |
| Arc flash | Voltage > 150 V dc and incident energy > 1.2 cal/cm² (5 J/cm²) at the working distance | Arc flash PPE per 130.7(C) |
§ Establishing an ESWC on an EDLC
370.4 is the heart of the article, and it is unusually prescriptive. Where a conductor or circuit part is connected to an EDLC system, a written procedure is required.
Documentation. The written procedure shall document the necessary steps and sequence to safely work on that part. The Informational Note notes this may involve discharging the EDLCs and placing the equipment into an electrically safe work condition.
Risk assessment. The written procedure shall incorporate the results of the risk assessment performed under 370.3(B) and be in accordance with 120.6.
EDLC discharge. Unless a discharge method is provided by the manufacturer, a guarded resistor rated to discharge the system shall be selected, and a method described to fully discharge it.
Discharge method. Shall include how to test for the absence of voltage, the expected discharge time, and what to do in the event there is still energy present.
Residual charge. Shall include a means to prevent residual charge build-up on the EDLC system after it has been discharged.
§ Why the residual charge requirement exists
“EDLCs can build up a residual charge without an external circuit.” An absence-of-voltage test that passes is not a permanent result. The charge can return on its own while the work is in progress — which is precisely why the standard demands a written procedure with an expected discharge time and a stated plan for what to do if energy is still present.
370.4(C) extends the same logic to storage and transport. Any residual charge shall be removed by discharging, and then:
- Uninstalled capacitors shall be short-circuited with a conductor sized to maintain the discharged state and to withstand mechanical abuse during storage and transport.
- An uninstalled capacitor found without its shorting conductor attached shall be treated as if charged to its full rated voltage until determined safe by a qualified person.
That second rule is a receiving-dock rule, not an electrical-room rule. A supercapacitor module that arrives, or is found on a shelf, without its shorting bar is a live part by definition — regardless of how long it has been sitting there. Most facilities have no procedure that treats a stored component as energized.
Supercapacitor ride-through is increasingly specified as an alternative to short-duration battery strings — it tolerates far more cycles and does not need the thermal management a VRLA or lithium string does. If your UPS has one, this article is yours and it did not exist in the prior edition.
The boundary matters: an EDLC ride-through module is Article 370, the UPS electronics around it are Article 330, the dc link capacitors are Article 320, and any battery is Article 360.
Check whether your ESWC procedure has an expected discharge time in it. For an EDLC system, 370.4(B)(2) requires one. Most procedures written for batteries or ordinary capacitors do not have that field at all.