Over current protection:overload and SC, circuit breakers and fuses

Monday, September 13, 2021
Veli Markovic

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 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


  • 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. 


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:





About the author

Veli Markovic

CEC Designer
Veli has nearly two decades of experience in the renewable industry. He is passionate about providing people with valuable education and is highly regarded throughout the industry as an educator and operator.
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