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Friday, 17 October 2014

Avoiding serious injury from an electric shock



We get an electric shock when current passes through our body due to a voltage difference. For example, if we touch a live wire at 230V, this voltage pressure will try and push current through our body to the ground that we are standing on. This is because the mains supply in a building is always at 230V with respect to the earth.


 
Effects of shock

The effect of the electric shock is dependent on the amount of current flowing through our body and also whether it is DC or AC voltage that is causing the shock. The table below gives more information on this.

Current (mA)
Effect
1 – 3
A slight tingling sensation
5 – 10
Painful.
10 – 16
Arm and hand muscles close involuntarily. Cannot let go.
20 – 25
Cannot breathe. Paralysis of chest muscles.
50 +
Heart fibrillation: Rapid, irregular contractions of the heart muscles. Could be fatal.

Residual Current Devices (RCDs)

From the above figures one can see that if the current can be kept low a fatal accident can be avoided. This is where RCDs come in useful.

RCDs monitor the current flowing through the Live and Neutral wires of an appliance. If there is any imbalance in this, say due to current flowing through a person to Earth, then they will cut off the mains supply to the appliance. They work at very low currents, and act very fast, typically in fractions of a second to protect the person from electrocution.

To protect a person from the risk of a fatal electric shock, RCDs need to operate at a current of less than 30mA.  

Contact resistance

The amount of current that flows through a person’s body is dependent on the voltage of the source and the contact resistance. The table below shows the amount of current that can flow in different situations.


Voltage Source
Type of contact
Typical resistance
Current (mA)
Effect
230V AC
Glancing contact with live part
500,000 ohms
0.46
A slight tingling sensation
230V AC
Grab live part with wet hands
10,000 ohms
23
Cannot breathe. Paralysis of chest muscles.

From this one can see the effect water has in reducing contact resistance. This is the reason that electrical sockets and switches are kept well away from areas close to water. This reduces the chance of a serious injury in case of a fault developing.

110V transformer



110V transformers are recommended when working with portable tools or on construction sites. They reduce the risk of a harmful electric shock significantly. They are centre tapped with the tap connected to earth. This makes the Live voltage only 55V with respect to earth. The table below shows the amount of current flowing when a 110V transformer is in use and there is contact with Live parts.

Voltage Source
Type of contact
Typical resistance
Current (mA)
Effect
110V transformer
Glancing contact with live part
500,000 ohms
0.11
Hardly noticeable.
110V transformer
Grab live part with wet hands
10,000 ohms
5.5
Painful

Summary

When working in environments which is damp and the chances of damage to appliances is high (outdoors, construction sites) then use of an RCD or a 110V transformer is essential to portect the user from serious injury from an electric shock.


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