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Household Electrical Guide: Electrical basics, Protection against Errors and Oversights continue…

Posted in Uncategorized on July 4, 2008

Electricity Fuse ratings

Formerly imperial ratings were used for fuses and circuit breakers, but the international Renard ratings have now been added and will supersede them.

Fuse manufacturers are still using the imperial sizes while circuit breaker manufacturers have mostly changed to the new ratings. An equivalence chart is shown above.

Miniature Electricity Circuit Breakers

The miniature circuit breaker (MCB) is now widely used and overcomes all the problems associated with ordinary fuses. It is a small, sophisticated unit affording a much higher level of protection than an ordinary fuse. It is tamper-proof and the unit involved is easily identified when one has tripped (the switch moves to the ‘off’ position). Most importantly, MCBs cannot be reset if the fault still exists; this eliminates the foolish and highly dangerous practice of putting in the wrong fuse wire or cartridge just to get things ‘conveniently’ working again for the time being.

MCBs are available in similar ratings to ordinary fuses and operate in two ways. Two fault conditions may arise: a short circuit and an overload on the circuit. A short circuit quickly increases the current flow through the unit. The solenoid coil (El) , therefore, increases its magnetic field and so attracts the moving core (E2) into the coil centre. This activates the trip bar (F) and causes the fixed contact (C) and moving contact (D) to open circuit. Arc runner (A) and arc chamber (B) act to suppress the arc formed on the contact point. The arc runner draws the arc across the arc chamber where it is broken up into small arcs which are quickly extinguished. The action of an MCB is much quicker than that of an ordinary fuse wire.

Appliances RenovationThe second type of fault, an overload on the circuit, would not cause the solenoid to trip. Instead, the current flowing through the thermo-metal (G) causes it to heat up. It consists of a tri-metal plate that bends when heated. The bending action activates the trip bar (F), causing fixed contact (C) and moving contact (D) to open circuit as before. This operation also is better than fuse wire and calibration to higher tolerances is possible.

MCBs are factory-calibrated to extremely accurate tolerances and must not be tampered with and cannot be repaired. The internal workings are shown here only as an aid to gaining an understanding of their operation and, thus, their usefulness. In the event of faults or failures, a completely new unit must be fitted.

Residual Electricity Current Devices

Neither fuses nor MCBs alone can give protection to anyone involved in a direct earth situation. This may also apply in the case of an indirect earth contact. The residual current device (RCD) has been developed to provide greater protection. It is also sometimes known as a residual current circuit breaker (RCCB) and, when it was first introduced, it was called an earth leakage circuit breaker (ELCB).

  • RCDs are available in various forms.
  • Mounted within the consumer unit to protect all or selected circuits.
  • As individual-socket protection.
  • An adapter to be used as portable protection and used where required.

The primary protection is the integrity of the earthing. In addition to the earthing, RCDs provide a much higher degree of protection depending upon the degree of sensitivity. For personal protection it is recommended that a sensitivity of 30mA is used.

It is generally considered that an earth fault of 1A or more is a fire risk. 50mA (50/1000 amp) or more risks causing a shock which can have varying effects upon the human body depending upon the value of earth fault current, the body resistance of the person and, of course, his or her state of health. The heartbeat cycle is about 0.75 second. It is vital to cut off the fault current in less than one cardiac cycle. The Wiring Regulations stipulate that for Indirect Contact protection isolation must occur within a maximum of 0.4 second.

How Electricity RCD works

An RCD protects by constantly monitoring the current flowing in the live and neutral wires supplying a circuit or an individual appliance. In normal circumstances the current flowing in the two wires is equal. When an earth leakage occurs through a fault or an accident, an imbalance occurs. This is detected by the RCD which automatically cuts off the power in a split second. Used correctly an RCD is an invaluable asset in any household.

To be effective, the RCD must operate very quickly and at a low earth fault current. Those most frequently recommended are designed to detect earth leakage faults in excess of 30mA (30/1000 amp) and to disconnect the power supply within 200 milliseconds (the rated sensitivity). These limits are well inside the required safety margin to protect against electrocution or fire.

An RCD is designed to sever mains current should an appliance malfunction electrically, or should you cut through or badly damage the mains cable of your lawnmower or vacuum cleaner for instance. It is simply a fail-safe device and should be used in addition to, not instead of normal overload protection, that is, fuses or MCBs.

All RCDs have a test facility and this should be used regularly. Devices used with adapters or sockets, or those for appliances designed for outside use, should be tested before each operation. If failure occurs (does not trip, or trip appears sluggish or hard to obtain) have the unit tested immediately. This will require an RCD test meter and is best left to a qualified electrician.

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