LV Protection
Fig 1:- SLD of a power system
Protection system is a branch of the electrical system that protects the system from faults by isolating the
fault part of the network. The main objective of the protective system is to ensure the operation of the
other part of the network by isolating the fault part. Basic idea of low voltage protection is the effective
protection of humans, animals, devices and network. LV protection devices are not as good as than MV
network due to that transferred power and interruption costs are much less that the MV network . The
low voltage network is very important component of the power system as it is at the level at which most of
the power is distributed and utilized by the end consumer.
For the protection of the LV system, the protection of the feeder is the most primary which is done through
the protection switchgears that are placed at the distribution substation. Each outgoing feeders(network)
have their own protection so that the fault in one network does not interrupt the supply to the others.
Generally the low voltage feeders are protected by a circuit breaker which trips according to the trip signal
sent by the relay which senses the voltage and current of the network via Voltage and Current Transformer
respectively
In a low voltage distribution network because of the diverse nature of the load connected it is generally
required that there is a bulk infeed to a LV switchboard and number of outgoing circuits. The air circuit
breaker(ACB) are generally preferred for the low voltage protection of the incomer because of its
simplicity, efficient performance, high normal current rating upto 600A and high fault withstanding
capacity up to 63kA. The outgoing circuits are generally connected with Moulded Case Circuit Breakers and
Miniature Circuit Breakers.
The most common protection device in LV network are low voltage circuit breakers,fuses (see below
product examples), switches, off load electrical isolators, earth leakage circuit breakers, miniature circuit
breakers, moulded case circuit breakers.
All the low voltage feeders that run out of the distribution substation are connected to different types of
loads. The loads may be residential load, lighting load, industrial load, motor loads and etc. Depending
upon the type of load that are connected to the system the protection system has to be selected.
Fig. 2 Distribution board and fuse-switch-disconnector [Source ABB Kabeldon]
Protection devices should be located at all circuits of transformer substation, where cable size changes and
end of the network consumer’s connection point.
Size of fuse shall be that it will be able to stand normal load but so as not to exceed the maximum
operating time when tripped. Protection devices shall be tripped fast enough also when 1-phase short
circuit occurs end of the network. If both of these requirements can not be met then is necessary to use
larger cable or to be placed intermediate-fuses.
Rating capacities of protection devices shall be take into account.
Selectivity of protection devices is always coordinated. So that when a fault occurs, only the fuse nearest a
fault trips without upstream fuses being affected.
Discrimination
The aim is total discrimination of whole system. The downstream fuse (or circuit breaker) trips as close as
possible to the fault, so that the upstream fuse (or circuit breaker) in not overloaded or trip. Discrimination
can be checked with tripping curves. The basic idea is that curves of protection devices not cut each other.
But also that protection device not overloads and at the same time trip fast enough when a fault occurs.
Fig. 3 Fuse discrimination (Source Schneider-electric & ABB)
Discrimination of fuses can be checked using I2/t curves. Downstream fuses I2/t value shall be lower than
upstream fuse’s pre-acting I2/t value.
Fig. 4 I2t values (Source ABB)
Fig. 5 Current and energy limiting (Source ABB)
Overload protection
Protection devices shall be coordinated with protected cables. Protection device shall be protecting the
cable so that cable is not damaged for example thermal or dynamic impacts.
The lowest single-phase short-circuit current in a distribution network shall be 2.5 x IN when gG fuse IN < 63
A and 3.0 x IN when gG fuse IN > 63 A. gG fuse intended to be used as an overload and short-circuit
protection of a conductor.
The maximum size of the gG fuse for AMKA cables depending ambient temperature. But if needed overload
protection of AMKA cables can be implemented also by fuses at the end of the line (customer’s main fuses).
In that case total rated of customer’s fuses may not exceed the load capacity of the AMKA cable.
Fig. 6 AMKA current ratings (Source Prysmian)
Underground cable network installations are in generally fireproof and due to this overload protection is
not required. Only the more important line may require better protection. (Consumer’s main fuse
protected overload installations after connection point.
Fig. 7 AXMK current ratings (source: Prysmian)
Short-circuit protection
The single-phase short-circuit current can be solved from
Aim of the protection is to disconnect in less than 5 seconds.
The maximum short circuit current that occurs at the installation point must be known prior to the
selection of the short circuit strength of the protective device.
So, it can be summarized as the low voltage system are protected against the short circuit and the overload
conditions by the use of electrical fuses and circuit breakers. However, there may be some case where
there we can encounter some internal faults, so a earth leakage circuit breaker is preferred for this scenario
which can detect a fault rating even of 100mA.