PreambleI have some test equipment (an HP6253a, for example) that I'd like to probe around inside while powered on in order to assist debugging. Since these equipment are powered by mains (US), this activity is inherently dangerous and can kill you under the right circumstances. A while back I took a high-voltage safety training course through my employer; with that training in mind as well as my other readings around this forum and the internet I have written down the following safety plan. Would the kinds folks on the forum take a look at my safety plan and let me know if I've missed something important?
Safety PlanSafety is achieved during any activity through both
passive and
active safety measures. Passive safety measures should be considered a last resort backup should any active safety measures fail.
Passive Safety MeasuresA test setup diagram is provided as an image below. Mains power is received by a 1:1 isolation transformer which is properly rated for the amount of power the device under test requires. The isolation transformer provides a single isolated output through a cheater plug with the third wire ground lifted to isolate the device under test from earth ground. The device under test shown is a notional example of a power supply; in this case both the internal circuitry and the metal chassis are floating. The device under test may or may not refer the internal circuitry to the metal chassis; in this diagram they are floating with respect to each other.
This test setup allows for a single fault without the risk of an electric shock from the device under test. A person inadvertently touching a single point of the internal circuitry will not close a loop for current to flow and therefore an electric shock will not occur. If the metal chassis is floating with respect to the internal circuitry then this test setup is double-fault tolerant in the specific case of a person touching both the metal chassis and a single point of the internal circuitry.
Additional passive safety measures could be taken but are not required, such as using properly rated insulating electrical work gloves or standing on a properly rated electrical isolation mat. Different applications or test environments may call for different or additional passive safety measures.
If a person is wearing an ESD wrist-strap, it must have a 1MOhm-10MOhm bleed resistor installed (rather than a direct short to earth ground).
Active Safety Measures- Never work on a live piece of hardware without fully understanding it, the dangers it presents, and where those dangers lie. Even while powered off, the internal circuitry may contain capacitors which hold dangerous level of charge for long periods of time.
- Always work on live equipment with one hand in your pocket. A single hand can still get shocked if two points in the internal circuitry are touched simultaneously, but at least the current is through the hand rather than the heart. A piece of equipment is live whenever it is plugged into mains.
- Always verify by test that there is electrical isolation between isolated-live to earth, isolated-neutral to earth, and chassis to earth. Never reference the device under test to earth ground; this means no single-ended oscilloscope probes.
- Always keep a clean and organized workspace. Always work slowly and deliberately.
- Never work alone.