Way to calculate the dangerousness level of volts+amps? (don't want to learn the hard way)

Question

Is there a formula to calculate the dangerousness of amps, and volts? like watts, for example. Watts = Volts x Amps. Generally speaking, you could say wattage takes into account volts, and amps.

I am working on a robot. And am using high power motors. I don't want to learn the hard way, by getting shocked.

I need a formula, or at least a wattage chart, to see, where, on the dangerousness scale my electrical system is.

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If I am touching a 1.2ft wire supplying 12v 80A of electricity, how fatal would that be (both heat wise, and shock wise), or would i be able to feel it at all?

Answer 1

NEC code puts a voltage limit of exposed conductors at 48 VDC or 34 VAC sinewave. Current is more of a hot-wire issue, not a shock issue. Volts times amps is watts, so if not a shock hazard it could be a burn hazard. A 25 watt solder iron will burn you as it is confined resistive heat. In this case the volts and amps that drive it are not so important.

Note that the NEC voltage limits assume worst-case conditions where a persons hands and feet could be wet, thus much more current could penetrate the body.

The following chart and paragraph are from wiki/High_voltage

The International Electrotechnical Commission and its national counterparts (IET, IEEE, VDE, etc.) define high voltage as above 1000 V for alternating current, and at least 1500 V for direct current—and distinguish it from low voltage (50 to 1000 VAC or 120–1500 VDC) and extra-low voltage (<50 VAC or <120 VDC) circuits. This is in the context of building wiring and the safety of electrical apparatus. In automotive engineering, high voltage is defined as voltage in range 30 to 1000 VAC or 60 to 1500 VDC.

I know someone who grabbed phase A and C of a 600 VAC 3-phase surge suppressor. The image of the copper bus bars are forever burned into the palms of each hand. How he survived we do not know. Because our body acts like a resistor, doubling the voltage puts 4 times the wattage through our body. 480 VAC hits 16 times as hard as 120 VAC.

Answer 2

Dangerous to YOU, and dangerous to circuits.

Changes in Magnetic fields---when you or your circuits are near that field.

Example: a long straight wire carrying high currents with rapid changes in current, near your PCB. Any "closed path", even if part of path is Ground and part of the path is analog sensor traces or analog amplifier traces or even digital MCU traces, will be a problem. Let us compute how big a problem.

V_induce = [MUo * MUr * Area/( 2 * PI * Distance ) ] / dI/dT

Let Area = 10cm by 10cm PCB (4" by 4")

Let Distance from a high-current wire, to yur PCB, also be 10cm

Let your dI/dT = 100 amps / 100 nanoseconds = 1 Billion amps/second

With MUo = 4 * pi * 1e-7 Henry/meter, MUr = 1 (air, copper, aluminum, FR-4)

The formula simplifies to

V_induce = (2e-7 * Area/Distance) * dI/dT

and substituting, we have

V_induce = (2e-7Henry/meter * (10cm * 10cm)/10cm) * 1+9

V_induce = 2e-7 * 10cm/1meter * 1e+9 == 2e-8 * 1e+9

Vinduce = 20 volts

Thus any circuits or sensors or logic signals, nearyour high_current motor wiring, are at risk of constant spiking at levels far above their rails, or far above their noise immunity.

CURE ?

Use large diameter TWISTED PAIRS to carry motor currents; the adjacency of HOT and RETURN high current (and high dI/dT currents) will greatly reduce the level of spikes we just computed.