1.5 hp dc motor

No-one would disagree with the advice to be very careful!

However, as a pedantic point of order, John has it the wrong way round - 230Vac is more dangerous than 230Vdc. Three reasons:

1. The current flow depends on the the applied voltage and the resistance of the victim (in ohms, not will-power). Mains voltage 230Vac is the RMS value (the average), and it peaks at about 320V. It can pass more current than 230Vdc, albeit in pulses. I=V/R
2. Humans are about 4 times more sensitive to AC current than DC.
3. 230Vdc causes a single muscle spasm, whereas AC causes many. A single spasm is more likely to throw you off.

It's best never to mess with electricity because the effects vary with voltage, frequency, individuals, and the nature of the contact. Rule of thumb says high voltages are more dangerous than low but it's unpredictable and/or contradictory. The lowest fatal shock I know of was from 32Vdc, admittedly in unlucky circumstances. DC is much more likely to arc than AC but 230Vdc is unlikely to arc to a human. In the USA they believe 110Vac to be safer than 230Vac systems because less current flows. Except muscle reaction to the shock means you're more likely to get stuck at 110Vac, so the current might flow for longer! In practice the two systems seem to be equally dangerous. Conversely, much higher voltages aren't always as lethal as might be confidently expected - electric chairs have failed to kill and people occasionally survive direct contact with the super-grid. Effects change with frequency as well. Radio Frequency power inflicts deep burns rather a shock. Is a shock from a VFD at full speed more dangerous than ordinary mains? No idea, and I'm not going to try it! Some people are far more sensitive to electricity than others which is why horseplay with a Megger is stupid.

Bottom line - better safe than sorry! Taking calculated risks with suitable precautions is fine, careless gambling isn't. Turning something on from a distance to test it is pretty safe compared with powering up and then poking around inside the same kit with a 6" nail and a belief in one's personal immortality...

Dave

Edited By SillyOldDuffer on 30/12/2018 17:13:19