Engineer's Notes: HV safety and switching
Every HV accident breaks a rule
Most serious HV incidents happen during switching and earthing: the wrong switch, earthing a cable still live from the far end, a detector that was never proved, or someone starting work before the permit. A written, checked switching program and strict order of operations stop almost all of them.
Isolate, lock, prove, earth, then permit
The order is not optional. Open and lock every source, check the voltage indicators, prove dead with a tested detector, apply earths, post notices, and only then issue the permit. Remove everything in reverse order after the permit is cleared and cancelled.
Arc flash is about time
Incident energy is roughly proportional to how long the arc lasts. Fast protection, arc flash relays, maintenance switches and remote racking reduce it far more than heavier PPE. Above 40 cal/cm², no PPE is adequate: the only safe option is to de-energise.
Know your local rules
Permit names, approach distances and authorisation levels differ between DEWA, Australian DNSPs, the New Zealand SM-EI and NFPA 70E. The principles are the same; the details are set by your network operator and employer.
Frequently asked questions
What are the five safety rules before working on HV equipment?
Disconnect completely, secure against reconnection (lock and tag), verify the absence of voltage with an approved detector, earth and short-circuit, and protect against adjacent live parts with barriers. Then issue the permit to work.
Why must you prove the voltage detector before and after testing?
A detector that has failed would show dead on a live conductor. Proving it on a known live source or proving unit before and after the test confirms it was working when it showed no voltage.
How is HV arc flash incident energy calculated?
IEEE 1584 gives empirical equations based on voltage, bolted fault current, arcing time, electrode gap, enclosure and working distance. Incident energy rises in direct proportion to arcing time, so faster protection is the most effective way to reduce it.
What is the arc flash boundary?
It is the distance from exposed live parts at which the incident energy of an arc would be 1.2 cal/cm2 (5 J/cm2), enough to cause a second-degree burn on bare skin. Anyone inside it needs arc-rated PPE.
What is the difference between a Permit to Work and a Sanction for Test?
A Permit to Work covers work on equipment that is isolated, proved dead, earthed and locked. A Sanction for Test is used when testing needs the earths removed or test voltage applied, for example a VLF cable test, and it suspends the permit for that equipment while it is in force.
Topics: switching program · five safety rules · proving dead · voltage detector · earthing switch · portable earths · permit to work · sanction for test · LOTO · approach distances · arc flash · IEEE 1584 · NFPA 70E PPE categories · arc flash boundary · SM-EI
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