Technical Journal
January 2013
Protection of LV Cables
Low voltage electrical networks without cable cannot be imagined. Power
cables are extensively used worldwide both with copper and aluminum.
Critical aspect of selection of cables depends on three major criteria.
Type of Installation
Thermal withstand capability
Short circuit withstand capability
The type of installation and thermal withstand are very much inter-related.
When cable starts carrying current, temperature starts increasing. Selection
of type of installation becomes very critical in ensuring heat dissipation.
Alternatively it becomes necessary that sizing of cable to be liberal such
that heat generated is taken care.
There are different methods of installation of cables which we will discuss
in the subsequent journals.
In this paper we will analyze protection of cables against short circuit.
When cable size is being selected, it is also necessary to obtain information
on withstand energy of the cable. Cable is protected only when protective
device is selected which is having its let through energy (I²t) lower or equal
to that of withstand energy of cables.
Where
I²t – Let through energy of protective device (For eg..MCB or MCCB)
k²S² – Withstand energy level of cable
S – Cross section of the cable (In case parallel cables are used, cross section
of one cable to be considered).
K – Factor depends on cable insulating and conducting material.
Protection of LV Cables
Discription
Initial temperature °C
Final temperature °C
Material of conductor:
Copper
Aluminium
PVC 300 mm² PVC >300 mm² EPR XLPE
Rubber
70
160
70
140
90
250
60
200
115
76
103
68
143
94
141
93
The table provides “k” factor values for different conducting materials as
well insulating materials. The value can also be arrived at with calculation
which we will discuss in the future.
When selection of cables is done it is advisable to implement above
formula along the length of the cable, However calculations at minimum
and maximum length of the cable will be sufficient in normal cases. The
highest short circuit point is at beginning of the cable (3-phase fault) and
lowest short circuit point is at the end of cable (Phase neutral or Phase to
ground).
The below chart provides typical values for withstand energy of cables.
However the values might vary from manufacturer to manufacturer.
Protection of LV Cables
In the below scenario, one can visualize protection of cables with moulded
case circuit breakers. In the curves, Red line represents cable characteristics
and Blue/Black represents let through energy curves of the protective
devices. As mentioned earlier let through curve of protective device below
the cable curve ensures protection of cable against short circuit. The curve
of protecting device above cable curve will not be protecting the cable.
Specific let through energy curve LLL
Specific let through energy curve LLL
Ikmax=3.0kA
1E4MA²s
1E4MA²s
MCCB 10A
Cable
Cable
1E3MA²s
1E3MA²s
100MA²s
100MA²s
10MA²s
10MA²s
1MA²s
1MA²s
MCCB 10A
0.1MA²s
0.1MA²s
1E-2MA²s
1E-2MA²s
1E-3MA²s
1E-3MA²s
1E-4MA²s
1E-4MA²s
1E-5MA²s
1E-5MA²s
1E-4kA
1E-3kA
1E-2kA
0.1kA
1kA
10kA
100kA
1E3kA
1E4kA
1E-4kA
1E-3kA
1E-2kA
0.1kA
1kA
10kA
100kA
This clearly indicates that proper care is required to be taken in selecting
the protective devices to protect the cables. The life of cables normally
varies from 10years to 30years in line with standards requirement. It is very
much important to select circuit breakers of quality of highest standards to
ensure proper protection for expensive cables.
It is recommended to use protection device manufacturer’s dedicated
software to select the circuit breaker of right choice and ensure to use
only those devices which are incorporated in the software
This method of protection of cables ensure maximum life cycle and
provides value for money (Cables being one of major capital expenditure in
a project).
1E3kA
1E4kA
Protection of LV Cables
ABB offers complementary software “DOC” which can be used to select
right protection device. The software offers versatile features in protection
devices selection as well as cable selection from 415V to 36kV AC network.
ABB offers comprehensive protection devices for cables which are used for
power distribution and motor control applications.
Contact us for further information
ABB Limited
Design Institute
88/3, 88/6, Basavanahalli Village
562123, Bangalore North, Karnataka, INDIA
email: Ramprasad.satyam@in.abb.com
Technical Journal 01/13