on May 6th, 2013

Dear Colleagues

No matter what your engineering discipline - you would have learnt about Ohm’s Law at some time or other (even at school). Herewith a simple application question which tests your conceptual knowledge.

Ohm’s Law
As we all (should) know, Ohm’s Law states that the current through a conductor between two points is directly proportional to the potential difference across the two points with the following formula:

I = V/R

where I is the current through the conductor in units of amperes (A), V is the potential difference measured across the conductor in units of volts (V), and R is the resistance of the conductor in units of ohms. (Thanks Wikipedia).

Do you Understand Ohm’s Law?
An electrician opens the control panel of a 3-phase resistive electric furnace while it is in operation and accidentally receives a shock from one of the 400 Volt lines inside the panel, while the furnace is drawing current (of say 100 amps). This is situation A.

The furnace is now switched off; but the electrician (being an idiot) accidentally touches the same place before (situation B) and gets another shock from the lines feeding the control panel. Assume everything is the same for both situations (skin resistance/point of contact/humidity).

Which was the more intense shock for the electrician ? Situation A or B ?

The Answer
The answer has to be Situation B where the voltage has gone higher due to lack of load (no current being drawn) of the furnace. The 100A current in the first situation A has no direct relevance (apart from being a red herring).

A few Parting Comments

  • The amount of current required to cause an electrical shock is tiny compared to the current drawn by the furnace.
  • The shock you receive is really dependent on your own resistance and the contact voltage. There is a higher voltage present for the second situation as there is no load.
  • A resistive furnace was referred to to avoid confusing with phase angles between current and voltage for an inductive furnace.
  • One should really consider peak voltages (not rms) as per above and line to ground (not line to line voltages); but this would make no difference to the answer.
  • Don’t make the mistake of thinking that because the first situation A has a 100 amps flowing through the circuit; that this level of current is likely to go through you if you touch a live part.
  • The resistance of the human skin can vary depending on moisture and open wounds; so the amount of current flowing through your body can vary. So being wet would cause a considerably greater shock than being dry. It only takes 20mA to stop your heart. Oddly enough, someone mentioned to me that he understood that higher currents won’t necessarily stop your heart but put you  into a convulsion thus sometimes helping you to shake yourself free from a high voltage source.

Thanks to John Reid for posing the question.

As Elbert Hubbard wryly observed: You can lead a boy or girl  to college but you can't make them  think.

Yours in Engineering Learning


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