When it comes to electricity, any kind - AC or DC - there are conductors and there are insulators. The human body, among other substances, lies somewhere in between. It is true that one would have a better chance of electrical shock with AC, Alternating Currents, because of the higher voltage. But, we must also learn to respect the power of low voltage sources such as the large capacity forklift batteries. While you are working with electric forklift controls, there is often the possibility of encountering an electric shock by touching the "live" conductors when the battery is still plugged in. The shock is a sudden involuntary contraction of the muscles, with a feeling of pain, caused by electrical current flowing through the body. If enough current is allowed to flow through the body it can be fatal. Safety first, therefore, should always be the rule!
The greatest shock hazard comes, no doubt, from high voltage circuits, but voltages as low as 30 volts can be dangerous. The amount of current that can flow through the human body depends on several factors, including the body resistance. The higher the body resistance the less current will be allowed to flow. Typically if you hold a conducting wire in each hand, the resistance of the body across the conductors is about 10,000 to 50,000 ohms. The higher the ohm reading, the higher the resistance and the lower the amount of current which can flow. Holding the conductors tighter may also reduce the overall resistance. Note that it is the current through the body - not through the circuit - which causes the electric shock.
As an example, 500 volts across a body resistance of 25,000 ohms produces 0.02 amps, or 20 ma, which can be fatal. Actually, as little as 10 micro amps through the body can cause a shock. Tests have shown that body resistance under certain conditions may be as low as 300 ohms and possibly as low as 100 ohms from temple to temple if the skin is broken. Experiments have been done to determine the amount of current at which a person can release himself from a live conductor, this "let-go" current was about 9 ma for men and 6 ma for women. This value was measured with AC current and the results for DC current were quite higher: 62 ma for men and 41 ma for women.
Another consideration is the amount of power being supplied from the battery source. If the voltage source can not supply enough power to deliver the full amount of current, without dropping in voltage, then it will be less able to shock an individual. When working with a forklift battery, be aware that it can supply plenty of current and will not reduce in voltage when suppling small currents through the body. Here is an idea of probable effects of shock from a DC source such as a forklift battery: 0-4 ma = perception; 4-15 ma = surprise; 15-80 ma = reflex action; 80-160 ma = muscular inhibition; 160-300 = respiratory block and over 300 ma of DC current can cause death.
Now that we know what can cause a dangerous shock lets see how we can prevent the experience:
1. Get a desk job with an assistant to do all your electrical work for you.
2. Always, and I do mean ALWAYS, disconnect the battery before replacing or removing defective components.
3. Always, and again I do mean ALWAYS, completely discharge large capacitors with a resistor or volt-ohm meter.
4. It is a good idea when taking measurements on any system to keep one hand in your pocket, so that you are not touching potentially different points with both hands. With one hand away from the vehicle it will greatly increase your resistance path and make any measurements safer.
5. Remove rings or other jewelry that may be a conductor and allow current to flow through them, if they should touch a "live" conductor. If current begins to flow through a ring, which is tight against your finger, it could get very hot, depending on the current flowing and cause severe burns.
Last but not least, just use common sense and take your time when working on any electrical device when power is applied. In most cases when we service low voltage forklift vehicles electric shock, which can cause death, is fairly remote. But there is always the potential for dangerous shocks or serious burns. Understand the situation at hand and proceed with safety in mind.
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