Over current protection:overload and SC, circuit breakers and fuses
September 13, 2021
By
Lahiru Sendapperuma
Protective device requirements
We look at the requirements of protective devices, circuit breakers and fuses, when it comes to addressing overloads and short circuits in the context of a commercial solar system.
Ties in with our prospective short circuit current presentation and is required information for CEC installers, designers and anyone interested in commercial solar systems, project management, sustainability, renewable energy and energy conservation.
Why is protection so important?
If specific current limits are exceeded overheating in the cable occurs:
This leads to excessive power loss in the cables
Insulation failure results due to excessive power loss
This heating effect is proportional
The heating effect
If specific current limits are exceeded overheating occurs
Heating effect is proportional to the square of the current
In case of 100% overload, heat loss 4 x times more than normal load
H= I^2Rt joules
Where H = heat produced in joules
I = conductor current in amps
R= resistance of conductor in ohms
t = time in which current flows in seconds
Massive overload
If overload is massive, as would occur with a short circuit of low impedance:
Protection must be capable of interrupting fault current BEFORE it can rise to a dangerous level
Protection has to disconnect without damage to itself ( very important)
Protection must have “decent” short circuit breaking capacity, CB’s, rupturing capacity, fuses
What are the requirements for protecting the wiring
Circuit Breakers and fuses:
Be able to carry rated current continuously without overheating or breaking down
Protection should not operate with small overloads for short periods of time
If small overload has lengthy duration, protection must kick in
When fault currents occur, protection must disconnect circuit immediately before damage occurs
Discrimination is required so only faulty circuit is isolated, no effect on other circuits
Overload protection
Overload protection:
Circuit opens before damage to cable
Overload action is relatively long
Time has inverse relationship with current, for example, 2 hours @ 125% overload, 3 seconds @ 600%
What about short circuit protection
Short circuit protection:
CB’s action has to be rapid to before damage occurs
The protective device can’t damage itself
High levels of energy dissipated in a short amount of time
The functions of overcurrent protection
Circuit Protection has two distinct main functions:
Protect wiring against overheating and breakdown
Interrupt supply quickly, limit value of energy available in the case of short circuit
Third point is discrimination. Happens when protective devices are in series
Overload Protection
Protective device deals with overload currents by:
Cutting off the flow before damage is caused to itself
To joins and insulation
Other materials surrounding the conductor
What do the Standards say?
2.5.3 Protection against overload current
2.5.3.1 Coordination between conductors and protective devices
The operating characteristics of a device protecting a conductor against overload shall satisfy the following two conditions:
IB <= IN <= IZ . . . 2.1
I2 <= 1.45 <= IZ . . . 2.2
Where:
IB = the current for which the circuit is designed, e.g. maximum demand
IN = the nominal current of the protective device
IZ = the continuous current-carrying capacity of the conductor (see the AS/NZS 3008.1)
I2 = the current ensuring effective operation of the protective device and may be taken as equal to either—
(a) the operating current in conventional time for circuit breakers (1.45 IN); or
(b) the fusing current in conventional time for fuses (1.6 IN for fuses in accordance with the IEC 60269 series)
An example
Let’s say we have a 95mm2 cable:
AS3008 and determine 95mm2 can carry, say 216A* so IZ = 216
Nominal current of the circuit breaker is 160A, so In = 160
Max demand designed for that circuit is 150A so IB = 150
So a 160A circuit breaker will suffice
*partially surrounded by thermal insulation, Table 15, column 11
Short circuit protection
What are the limits:
In the case of a short circuit the only limit to the current is the impedance of the circuit
The available short circuit energy
Usually includes the impedance of the supply source
Usually a substation transformer or group of transformers.
Conclusion
Excessive heat in a cable can lead to potential safety issues and protective devices must cope with overload and short circuit currents. Must know cable and protection specs and how they relate to max demand of the circuit.
If you’d like to see more of what Greenwood Solutions get up to in the real world of renewable energy, solar, battery storage and grid protection check out the following pages:
Lahiru's spent 15 years in the energy industry, driving outcomes and twin cabs. A good chunk of his Greenwood hours are spent onsite, overseeing the delivery of utility projects and solar farms. And when he punches the clock, Lahiru's still tackling challenges (wearing the uniform of his local NFL club).
