Introduction – to Earth Leakage and the Need to Protect Against it
L. B. SinhaL. B. Sinha, G.M.(Training & Audit), ASK-EHS Engineering & Consultants Pvt. Ltd., Surat, Gujarat.
In the context of electricity, for its proper conservation, right distribution to the desired points of utility and also to safeguard the precious life and valuable property from damage, it is very important to insulate the point of voltage and the passage of electric current through the conductor. Every student and user of electricity must clearly understand and appreciate the importance and role of sound insulation which guides the correct passage of electricity through desired conductor. Any leakage of current, besides being a loss of electrical energy, may be a potential danger to lives of human beings and cattle flock, examples of which we find in plenty in day-to-day life. Large animals, besides human beings, lose their lives due to electrocution when a very small amount of current, in the order of milli-Amperes, flows through their bodies on account of leakage of current from the potential point.

Human safety is the worst victim in case of weakening of insulation because the deteriorated strength of insulation brings harmful potential close to the human body, increasing the chances of flow of intolerable extent of current. This is in brief, the perspective of electrical leakage due to deterioration in fortification of electric power with network of varieties of insulating materials and gadgets.

Earth Leakage Protection

It must be clearly understood and always born in mind that earth leakage protection device is not the surest way of saving human life. As can be seen from the analysis of the subject electrical network, there may be many instances and situations wherein man can be subjected to harmful voltage, in spite of ELCB being in place and active in operation. Hence, the need to be realistic and away from the false notion that we are always safe with the use of ELCB.

Human Tolerance Limit

Human tolerance limit for electrical currents
A body resistance model would indicate approximately 1,000 Ω from hand to hand, or about 1,100 Ω from hand to foot. It is believed that the greatest number of injurious shocks involve a current pathway that is either hand to hand or hand to feet, because the vital parts such as heart and lungs encounter the flow of current.

A correctly chosen and designed ELCB which can detect small currents of the order of 0- 10 m A flowing to earth can reduce the risk of electrocution. Effect of electric current through human body has been well researched and following chart summarizes the results.

Limitations of ELCBs from practical point of view
An earth leakage circuit breaker cannot remove all risks of electric shock or fire. In particular, an ELCB alone will not detect overload conditions, phase to neutral short circuits or phase-to-phase short circuits. Over-current protection (fuse or circuit breaker) must be provided. Circuit breakers that combine the functions of an ELCB with overcurrent protection respond to both types of fault. These are known as Residual- Current Circuit Breaker with Overcurrent protection (RCB0s), and are available in 1, 2, 3 and 4 pole configurations. RCBOs will typically have separate circuits for detecting current imbalance and for overload current but will have a common interrupting mechanism.

An ELCB will help to protect against electric shock where current flows through a person from a phase (live/line/hot) to earth. It cannot protect against electric shock where current flows through a person from phase to neutral or phase to phase, for example where a finger touches both live and neutral contacts in a light fitting; a device can not differentiate between current flow through an intended load from flow through a person.

Situations where conventional ELCBs will not be able to protect human life
The safe level at which the leakage current can pass through the human body without severe shock or injury is estimated to be 4 to 6 milli-Amperes with a duration of less than 25 milli-seconds. However, most of the electrical appliances in use in the homes and offices today do have certain degree of electrical leakage current and when they are used together with the same circuit breaker; the total leakage current could easily exceed the predetermined level of the circuit breaker.

This phenomenon poses a problem in selecting the predetermined level of leakage current, since if the electrical leakage current is set to a safe level of say, below 10 milli-amperes, to ensure the safety of the user, then the circuit breaker is rendered useless as it would trip frequently when several electrical appliances with normal leakage current are used simultaneously. Conversely, if the pre-determined level of leakage current is set at a higher level to enable the electrical appliances with normal leakage current to be used, then the electrical leakage current setting would have exceeded the safety limit the human body could endure.

With such a dilemma, it is difficult to design the circuit breaker to protect the user from severe electrical shock or injuries as well as to provide a safe limit for all the electrical appliances in use. Moreover, with the increasing number of electrical appliances in use in the homes and offices, the risk of the user receiving an electrical shock is higher. Such increase in the use of electrical appliances also means that a setting of higher electrical leakage current is required in order to prevent frequent tripping of the circuit breaker even when there is no electrical earth fault in any of the appliances in use.

Situations where ELCBs can protect human life
The safety level at which the leakage current can pass through the human body without severe shock or injury is estimated to be 5 ±1milli-Amperes with a duration of less than 1/40 seconds (i.e. 25 milli-seconds). Suppose leakage current is 30 mA, in this case an Indian made 30 mA ELCB easily senses the current and is tripped and supply to load is interrupted.

Principle of operation
The ELCB 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 ELCB 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.

In the United States, the National Electrical Code, requires GFCI devices intended to protect people to interrupt the circuit if the leakage current exceeds a range of 4-6 mA of current (the trip setting is typically 5 mA) within 25 milliseconds. GFCI devices which protect equipment (not people) are allowed to trip as high as 30 mA of current. In Europe, the commonly used RCDs have trip currents of 10-300 mA for machine and fire protection.

Residual current detection is complementary to over-current detection. Residual current detection device cannot provide protection for overload or short-circuit currents.

RCDs with trip currents as high as 500 mA are sometimes deployed in environments (such as computing centers) where a lower threshold would carry an unacceptable risk of accidental trips. These high-current RCDs serve more as an additional fire-safety protection than as an effective protection against the risks of electrical shocks.

In some countries, two-wire (ungrounded) outlets may be replaced with three-wire GFCIs to protect against electrocution, and a grounding wire does not need to be supplied to that GFCI, but the outlet must be tagged as such. The GFCI manufacturers provide tags for the appropriate installation description.

Earth Leakage Circuit Breaker

The photograph depicts the internal mechanism of an Earth Leakage Circuit Breaker (ELCB). The device pictured is designed to be wired in-line in an appliance flex. It is rated to carry a maximum current of 13 amperes and is designed to trip on a leakage current of 30 mA. This is an active ELCB; that is, it doesn't latch mechanically and therefore trips out on power failure, a useful feature for equipment that could be dangerous on unexpected re-energisation.

The incoming supply live and the grounded neutral conductors are connected to the terminals at (1) and the outgoing load conductors are connected to the terminals at (2). The earth conductor (not shown) is connected through from supply to load uninterrupted.

When the reset button (3) is pressed the contacts ((4) and hidden behind (5)) close, allowing current to pass. The solenoid (5) keeps the contacts closed when the reset button is released.

The sense coil (6) is a differential current transformer which surrounds (but is not electrically connected to) the live and neutral conductors. In normal operation, all the current down the live conductor returns up the neutral conductor. The currents in the two conductors are therefore equal and opposite and cancel each other out.

Any fault to earth, for example caused by a person touching a live component in the attached appliance, causes some of the current to take a different return path which means there is an imbalance (difference) in the current in the two conductors (single phase case), or, more generally, a nonzero sum of currents from among various conductors (for example, three phase conductors and one neutral conductor).

This difference causes a current in the sense coil (6) which is picked up by the sense circuitry (7). The sense circuitry then removes power from the solenoid (5) and the contacts (4) are forced apart by a spring, cutting off the electricity supply to the appliance.

The device is designed so that the current is interrupted in a fraction of a second, greatly reducing the chances of a dangerous electric shock being received.

The test button (8) allows the correct operation of the device to be verified by passing a small current through the orange test wire (9). This simulates a fault by creating an imbalance in the sense coil. If the ELCB does not trip when this button is pressed then the device must be replaced.

Having studied the functioning of ELCB for the monitoring of earth leakage current which in other words indicates the health of deployed insulation in the electrical network, it is worth noting and further probing the following aspects of earth leakage situation vis-a-vis human safety, particularly in the context of Indian scenarios.
  1. As the very name signifies, ELCB has the primary function of monitoring the health of insulation by measuring the extent of leakage through the used insulation. The user of ELCB sets a tolerable value of leakage current, mostly setting at 30 mA tripping current. That means that we are prepared to tolerate a leakage current to the ground to the extent of 30 mA- which results in power loss at say 230 V to the extent of nearly 7 Watts {230x(30/1000)} which we can bear with. Indirectly, it shows that the applied insulation which generally in healthy and dry condition is of the order of a few meg-ohm has suffered a deterioration to the extent of low value of barely about 8000 ohm. As an electricity user it is a fair tolerance level accepted, beyond which the situation demands that we should inspect and investigate as to where the insulation has suffered a set-back and thereafter take immediate action to improve the insulation level before switching on the circuit and take power out of it. ELCB does its assigned task pretty well upto this extent.
  2. Having done so pretty well its primary duty as mentioned above, let us see how it fairs in tackling a secondary notional task of saving human life which is much more important than accepting a power loss of a few watts, say 7 W. It is well-known that the extent of current flow through human body, particularly through its vital parts such as heart and lungs, should not exceed 10 mA. Therefore, a very common-sense suggestion would be to have a tripping device in the order of 5-10 mA - better 4-6 mA which is very much prevalent in US. For saving valuable human lives in India as well, there is a definite need for all the manufacturers and users of ELCB to rise to the occasion and popularize the application of high sensitivity ELCB in the tripping range of 4-6 mA with 25 milli-second tripping time. Only such high sensitivity device can be claimed as actual saver from detrimental level of earth leakage current. Such preferred values have been defined by the International Electro technical Commission (IEC) for the specific purpose of life injury protection.
General complaints have been put forward that such high sensitivity devices trip quite often due to risen value of capacitative leakage current to the ground - causing annoyance and giving a feeling of nuisance tripping. If we actually analyse the prevailing untidy electrical installations, particularly at the temporary project sites, such nuisance/frequent trippings are because of unsound insulation conditions and due to use of large lengths of wires/ cables at a stretch. In all prudent sense, the matter needs serious consideration, if at all we claim to save human life and care for human beings, otherwise, ELCB will be left to provide human safety under very limited situations. We are required to draw lessons from the practices in USA and the preferred recommendation of International Electrotechnical Commission (IEC).

This is a safety call to the users of electricity and the manufacturing sector of ELCB in India.
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