An Earth Leakage Circuit Breaker is a safety device used in electrical installations with
high earth impedance to prevent shock.
It detects small stray voltages on the metal enclosures of electrical equipment, and interrupts
the circuit if a dangerous voltage is detected.
Once widely used, more recent installations instead use residual current circuit breakers
which instead detect leakage current directly.
Purpose The main purpose of earth leakage protectors
is to prevent injury to humans and animals due to electric shock.
History ELCBs were mainly used on TT earthing systems.
Nowadays, ELCBs have been mostly replaced by residual-current devices.
However many ELCBs are still in use.
Early ELCBs responded to sine wave fault currents, but not to rectified fault current.
Over time, filtering against nuisance trips has also improved.
Early ELCBs thus offer a little less safety and higher risk of nuisance trip.
The ability to distinguish between a fault condition and non-risk conditions is called
discrimination.
ELCB manufacturers include: Major Tech, Fuji Electric, Schneider Electric, Legrand, Havells,
ABB, Siemens AG, Areva T&D, Camsco, Telemecanique, Orion Italia, Crabtree, MEM, Terasaki.
Types There are two types of ELCB:
voltage operated and, current operated.
Voltage-operated Voltage-operated ELCBs were introduced in
the early 20th century, and provided a major advance in safety for mains electrical supplies
with inadequate earth impedance.
V-ELCBs have been in widespread use since then, and many are still in operation but
are no longer installed in new construction.
A voltage-operated ELCB detects a rise in potential between the protected interconnected
metalwork and a distant isolated earth reference electrode.
They operate at a detected potential of around 50 volts to open a main breaker and isolate
the supply from the protected premises.
A voltage-operated ELCB has a second terminal for connecting to the remote reference earth
connection.
The earth circuit is modified when an ELCB is used; the connection to the earth rod is
passed through the ELCB by connecting to its two earth terminals.
One terminal goes to the installation earth CPC, and the other to the earth rod.
Disadvantages of the voltage-operated ELCB are the requirement for a second connection,
and the possibility that any additional connection to earth on the protected system can disable
the detector.
Current- sensing protection Residual-current devicess protect against
earth leakage using a different method of detection.
Both circuit conductors are run through a sensing coil; any imbalance of the currents
means the magnetic field does not perfectly cancel.
The device detects the imbalance and trips the contact.
When the term ELCB is used it usually means a voltage-operated device.
Similar devices that are current operated are called residual-current devices.
However, some companies use the term ELCB to distinguish high sensitivity current operated
3 phase devices that trip in the milliamp range from traditional 3 phase ground fault
devices that operate at much higher currents.
Operation An ELCB is a specialised type of latching
relay that has a building's incoming mains power connected through its switching contacts
so that the ELCB disconnects the power in an earth leakage condition.
The ELCB detects fault currents from live to the earth wire within the installation
it protects.
If sufficient voltage appears across the ELCB's sense coil, it will switch off the power,
and remain off until manually reset.
A voltage-sensing ELCB does not sense fault currents from live to any other earthed body.
Advantages ELCBs have one advantage over RCDs: they are
less sensitive to fault conditions, and therefore have fewer nuisance trips.
Therefore by electrically separating cable armour from the cable circuit protective conductor,
an ELCB can be arranged to protect against cable damage only, and not trip on faults
in downline installations.
Disadvantages ELCBs have some disadvantages:
They do not detect faults that don't pass current through the CPC to the earth rod.
They do not allow a single building system to be easily split into multiple sections
with independent fault protection, because earthing systems are usually bonded to pipework.
They may be tripped by external voltages from something connected to the earthing system
such as metal pipes, a TN-S earth or a TN-C-S combined neutral and earth.
As with RCDs, electrically leaky appliances such as some water heaters, washing machines
and cookers may cause the ELCB to trip.
ELCBs introduce additional resistance and an additional point of failure into the earthing
system.
Earth bypassing It is not unusual for ELCB protected installation
to have a second unintentional connection to earth somewhere, one that does not pass
through the ELCB sense coil.
This can occur via metal pipework in contact with the ground, metal structural framework,
outdoor home appliances in contact with soil, and so on.
When this occurs, fault current may pass to earth without being sensed by the ELCB.
Despite this, perhaps counterintuitively, the operation of the ELCB is not compromised.
The purpose of the ELCB is to prevent earthed metalwork rising to a dangerous voltage during
fault conditions, and the ELCB continues to do this just the same, the ELCB will still
cut the power at the same CPC voltage level.
Nuisance trips While voltage and current on the earth line
is usually fault current from a live wire, this is not always the case, thus there are
situations in which an ELCB can nuisance trip.
When an installation has two connections to earth, a nearby high current lightning strike
will cause a voltage gradient in the soil, presenting the ELCB sense coil with enough
voltage to cause it to trip.
If the installation's earth rod is placed close to the earth rod of a neighbouring building,
a high earth leakage current in the other building can raise the local ground potential
and cause a voltage difference across the two earths, again tripping the ELCB.
Close earth rods are unsuitable for ELCB use for this reason, but in real life such installations
are sometimes encountered.
Both RCDs and ELCBs are prone to nuisance trips from normal harmless earth leakage to
some degree.
On one hand ELCBs are on average older, and hence tend to have less well developed filtering
against nuisance trips, and on the other hand ELCBs are inherently immune to some of the
causes of false trips RCDs suffer, and are generally less sensitive than RCDs.
In practice RCD nuisance trips are much more common.
Another cause of nuisance tripping is due to accumulated or burden currents caused by
items with lowered insulation resistance.
This may occur due to older equipment, or equipment with heating elements, or even wiring
in buildings in the tropics where prolonged damp and rain conditions can cause the insulation
resistance to lower due to moisture tracking.
If there is a 30 mA protective device in use and there is a 10mA burden from various sources
then the unit will trip at 20 mA.
The individual items may each be electrically safe but a large number of small burden currents
accumulates and reduces the tripping level.
This was more a problem in past installations where multiple circuits were protected by
a single ELCB.
Heating elements of the tubular form are filled with a very fine powder that can absorb moisture
if the element has not be used for some time.
In the tropics, this may occur, for example if a clothes drier has not been used for a
year or a large water boiler used for coffee etc. has been in storage.
In such cases, if the unit is allowed to power up without RCD protection then it will normally
dry out and successfully pass inspection.
This type of problem can be seen even with brand new equipment.
Failure to respond Some ELCBs do not respond to rectified fault
current.
This issue is the same in principle with ELCBs and RCDs, but ELCBs are on average much older
and specs have improved considerably over the years, so an old ELCB is more likely to
have some uncommon fault current waveform that it will not respond to.
With any mechanical device, failures occur, and ELCBs should ideally be tested periodically
to ensure they still work.
If either of the earth wires become disconnected from the ELCB, it will no longer trip and
the installation will often no longer be properly earthed.
See also Residual-current device
Earthing system References