Protection
Against electroction
The use of exposed, substandard, badly wired, wrongly connected or damaged equipment as well as frayed or badly repaired cables reduces the safety of an installation and inreases the risk of person receiving an electric shock.
Electrocution is a passage of current through human body, which is dangerous. The flow of current through human body effects vital functions.
1. Breathing
2. Heartbeat
A correctly chosen RCCB can detect small currents flowing to earth and reduces the risk of electrocution. Effect of electric current through human body has been well researched and following chart summarizes the results:
Effect of electric current through human body has been well researched and following chart summarizes the results:
| 500 mA |
 |
 |
Immediate cardiac arrest resulting in death |
| 70-100 mA |
Cardiac fibrillation; the heart begins to vibrate and no longer beats at a steady rate. This situation is dangerous since it is irreversible |
| 20-30 mA |
Muscle contraction can cause respiratory paralysis |
| 10 mA |
Muscle contraction : the person remains “stuck” to the conductor |
| 1-10 mA |
Prickling sensations |
However, electrocution should not be viewed in terms of “current” alone but in terms of “contact voltage”. A person gets electrocuted by coming in contact with an object that has a different potential from his/her own. The difference in potential causes the current to flow through the body.
The human body has known limits:
- Under normal dry conditions, voltage limit = 50V
- In damp surroundings, voltage limit = 25V
Against indirect contact
Over current protection devices like MCB are unable to act promptly on small earth leakage currents. To comply with wiring regulations the earth fault loop impedance in Ohms, multiplied by the rate tripping current of the RCD in amperes must not exceed 50.
Example
For an RCD with a rated tripping current of 30mA, the maximum permissible earth fault loop impedance is calculated as follows: Zs (max) = 50 / In = 50/0.03 = 1,666
| Rated tripping current of the RCD |
Maximum permissible earth fault loop impedance in |
| 10mA |
5,000 |
| 30mA |
1,666 |
| 100mA |
500 |
| 300mA |
166 |
Against fire
The majority of fires which occur as a result of faulty wiring are started by current flowing to earth. Fire can be started by fault current of less than 1 amp.
The normal domestic overload protective device such as a fuse or MCB will not detect such a small current. A correctly chosen RCD will detect this fault current and interrupt the supply, hence reducing the risk of a fire starting.
Working principlew
The RCCB works on the current balance principle. The supply conductors, i.e., the phases and the neutral, are passed through a torroid and form the primary windings of a current transformer. Its secondary winding is connected to a highly sensitive electromagnetic trip relay, which operates the trip mechanism.
In a healthy circuit, sum of the currents in phases, is equal to the current in the neutral and the vector sum of all currents is equal to zero. If there is any insulation fault in the current and leakage current flows to earth, the currents do not balance and their vector sum is not equal to zero. This imbalance is detected by the core balanced current transformer, the RCCB is tripped and supply to load is interrupted. The trip mechanism is operated at a residual current between 50-100% of its rated tripping current.
Selection
30 mA
A 30 mA ELCB will provide a high degree of protection against electrocution in a accidental shock hazard situation. The current f lowing throgh human body could be between 80mA and 240mA depending on the resistance of the human body and the voltage across it.
| Zone |
Physiological Effects |
| Zone 1 |
Usually no reaction effects |
| Zone 2 |
Usually no harmful physiological effects |
| Zone 3 |
Usaully no organic damage to be expected. Likelihood of muscular contraction and difficulty in breathing, reversible disturbances of formation and conduction of impulse in the heart and transient cardiac arrest without ventricular fibrillation increases with current magnitude and time. |
| Zone 4 |
In addition to the effects of Zone 3, probability if ventricular fabriliation increased upto 5% (curve C2) upto 50% (curve C3) and above 50% beyond curve C3. It increases with magnitude and time, and pathophysiological effects such as cardiac arrest, breathing arrest and heavy burns may occur. |
To be within zone of the IEC curve as shown above. It is necessary for the ELCB to operate within 50ms at 240 mA and 150ms at 80mA. Both these conditions are satisfied by 30mA ELCB
For households, Individual outlets, Wet areas and temperary installations, ELCB with sensitivity not exceeding 30mA is advisable
100 mA
A 100mA ELCB will normally give high degree of protection against electrocution but there is a possibility that the shock current could fall below the tripping level of ELCB. This could occur if additional resistances to that of human body are included in the earth path.
The 100mA RCCB protects against leakage currents and indirect contact with earth loop impedance upto 500 Ohms.
300 / 500mA
A 300/500 mA ELCB may be used where only fire protection is required. eg., on lighting circuits, where the risk of electric shock is small. 300/500mA ELCB will not give any protection against electrocution.
Actuation time characteristics
Wiring diagram
The Greegoo's range of four pole RCBs can be used to provide residual current protection in 3 phase, 3 wire circuits (no neutral), however a link from the neutral to an incoming should be made on the supply side of the RCCB, to enable the operation of the RCCB.
|
