Last month Stewart concluded with the Let Go value of electric shock The body reaction to these currents will depend on the particular bodily makeup for example men, women and children. For example the average perception level for men is approx 1milli-amp but in the case of women the value is lower at 0.7 milli-amps. This is the value of current flowing in the body which will cause a tingling sensation to be felt. The average Let Go value for men is in the region of 10 milli-amps and for women slightly lower at 8 milli-amps. The following table shows the relative magnitude and effects of electric current on the body.

Note DEATH can be caused by any of the currents denoted by an *.

Ventricular Fibrillation is nearly always fatal, and occurs when an electric current passing through the chest affects the operation of the heart. The heart muscles are thrown out of syncronisation, regular pumping action ceases and the heart flutters for a short period and therefore no blood is pumped around the system. If the blood circulation stops the body tissue will be starved of Oxygen which would normally be supplied by the blood. In this case mouth to mouth on its own will be fruitless as all that will be happening is that the lungs will be inflated and de-flated but because the blood is not flowing no Oxygen will be absorbed and transmitted to the tissue via the blood stream. Cardiac Massage will be required to start the heart in conjunction with Mouth to Mouth. (Information and training on these techniques can be obtained from various sources including qualified First Aiders and of course the Internet).

IT IS GOOD TO KNOW

Limiting The Seriousness of Electric Shocks.

There are three basic precautions which can be taken to limit the effect of electric shock on the body these are:- 1 Increasing the resistance to limit the flow through the body. 2 Limit the Pressure to a safe voltage that cannot overcome the resistance (Remember Ohm's Law). 3 Limit the length of time that the current flows. Let us now consider these statements.

Increasing the Resistance to Limit Flow.

The average body resistance for a hand to foot electric shock at 240v is in the order of 2000 ohms (2k) So if we now substitute these values into Ohms Law we obtain:- I = V/R = 240/2000 = 0.12 AMPS (120 MILLI-AMPS)

As you can see this value is well in excess of the fatal levels stated in the previous table. If the same person were to stand on an insulating mat and wear insulating gloves each of which have 1,000,000 ohms (1m ohm) resistance we can now prove that the current would be reduced to a safe level.

I = V/R = 240/2002000 = 0.00012 AMPS (120 micro-amps)

Limiting the Pressure.

Again referring to Ohm's Law if the voltage is sufficiently low there will not be sufficient power to push an electric current thorough the body. This factor is used to advantage with low power hand tools as used on construction sites etc. A transformer is used to reduce the mains supply voltage down to 110v (still a lethal voltage) and then the centre tap is taken to earth, which will effectively reduce the major hazard of an electric shock (conductor to earth voltage 55 volts) which will in most instances be insufficient to cause a lethal shock.

Limiting the Time Current Flows.

Some type of Residual Current Device (R.C.D.) in a circuit will monitor current flowing to and from a piece of electrical equipment and if an imbalance occurs in excess of the specified trip level the unit will isolate the circuit. Most available units are designed to trip within 30 milli-seconds at a current of 30 milli-amps, from this it can be observed that in most instances although an electric shock of fatal current level will be received the time duration will not be great enough to prove fatal. WHY? Normally it will require the interfering signal (electric shock) to the heart to be of a greater duration than of half a cycle of the heart beat. This would normally be around 428 milli- seconds in a healthy person (assuming a heart beat of 70 beats per minute) to upset the synchronisation.