What does “Discrimination” mean in electrical circuit protection?

The goal of discrimination or protection coordination is to ensure that only the faulty part of a circuit is de-energised by tripping of the protective device.

What are different types of RCDs?

Standard IEC 60755 (General requirements for residual current operated protective devices) defines three type of RCD depending on the characteristics of the fault current; type AC, type A and type B. type AC is ensured for residual sinusoidal alternating currents. Types A, and B are used for residual currents which may result from rectifying circuits.

What is the range of sensitivities of RCDs?

RCD sensitivity is expressed as the rated residual operating current, noted IΔn. Preferred values have been defined by the IEC, thus making it possible to divide RCDs into three groups according to their IΔn value; high sensitivity HS (6, 10, 30 mA), medium sensitivity MS (0.1, 0.3, 0.5, 1 A) and low sensitivity LS (3, 10, 30 A).

RCDs for residential or similar applications are always high or medium sensitivity. It is clear that high sensitivity is most often used for direct-contact protection. Whereas MS and in particular 300 and 500 mA ratings are indispensable for fire protection. The other sensitivities (MS and LS) are used for other needs such as protection against indirect contacts (mandatory in the TT systems) or protection of machines.

What is the S type RCD?

RCDs are divided into two groups according to their break times; type G and S.

  • G (general use) types are instantaneous RCDs (i.e. without a time delay)
  • S (selective) types are RSDs with a short time delay.

Is it usual to use RCD protection for 24 VDC circuits?

No it is not usual, but refer to AS3000 for direction.

How can discrimination be achieved in RCD coordination?

There are two type of discrimination; vertical and horizontal.

Vertical Discrimination
This type of discrimination concerns the operation of two protective devices installed in series on a circuit. Given the tolerances around the RCD thresholds and break times, both current and time discrimination are used.

  • Current Discrimination

According to the standards, an RCD must operate for a fault current between IΔn/2 and IΔn. In fact the factor of three is required between the settings of two RCDs to avoid simultaneous operation of the two devices, i.e. IΔn (upstream) > 3 IΔn (downstream).

  •  Time Discrimination

Time discrimination is important for cases where the fault current suddenly exceeds both rated operating currents. It is necessary to take into account the response time, even minimal, of all mechanism, to which it may be necessary to add deliberate time delays. The double condition to ensure non-tripping of Da (upstream RCD) for the fault downstream of Db (downstream RCD) is:

IΔn (Da) > 3 IΔn (Db)
tr (Da) > tr (Db) + tc (Db) = tf (Db)


    • tr : non-actuating time


    • tc : disconnection time between the instant the operating order is given by  the measurement relay to the instant of disconnection (including the arcing time)


  • tf : break time, from detection of the fault through to complete interruption of the fault current; tf = tr + tc

The threshold detection circuits of electronic relays may exhibit a fault memorisation phenomenon. It is therefore necessary to take into account a “memory time”, that can be thought of as a virtual increase in the time that a current flows, to ensure that they do not operate after opening of the downstream device.

Note: particular attention must be paid when determining condition for circuit-breakers which add-on RCDs and residual-current relays used together. This is because a circuit breaker with an add-on RCD is defined in terms of the non-actuating time (tr). A residual-current relay is defined in terms of the time between the instant the fault occurs and transmission of operating order, to which it is necessary to add the response time of the breaking device. It is therefore necessary to calculate the successive tf and tr times (at 2 IΔn, the conventional current for the non-operating test of delayed RCDs) for each RCD, from downstream to upstream.

Horizontal Discrimination
Sometimes referred to as circuit selection, stipulated in the standard NFC15-100, section 535.4.2, it means that an RCD is not necessary in a switchboard at the head of the installation when all the outgoings circuits are protected by RCDs. Only the faulty circuit is de-energised. The RCDs placed on the other circuits (parallel to the faulty one) do not detect the fault current. The RCDs may therefore have the same tr settings. In practise, horizontal discrimination may present a problem. Nuisance tripping has been observed, particularly on IT systems and with very long cables (stray capacitance in cables) or capacitive filters (computers, electronic systems, etc.).

Which type of leakage currents can disturb the RCD operation?

There are a number of types of leakage currents likely to disturb RCD operation:

  • Leakage currents at power frequency
  • Transient leakage currents
  • High-frequency leakage currents

These currents may be natural, flowing through the capacitance distributed throughout the cables in the installation, or intentional, i.e. the current following through components used intentionally, namely capacitive filters installed on the supply circuits of electronic devices.

Is there any limitation in the number of devices connected to a RCD?

For a single-phase device in a 50 Hz system, continuous leakage currents of approximately 0.5 to 1.5 mA per device are measured. These leakage currents add up if the devices are connected to the same phase. If these devices are connected to all three phases, the current cancel out when they are balanced. Because of these leakage currents, the number of devise that can be connected downstream of an RCD is limited.

Given the RCD tripping may take place starting at 0.5 IΔn, it is advised, in order to avoid nuisance tripping, to limit the continuous leakage current to 0.3 IΔn for TT and TN systems and to 0.17 IΔn for an IT system. use an RCD with a narrow operating range (0.7 IΔn to IΔn) reduces the constraint.

Can RCD protection be used with variable speed drives?

High-frequency leakage currents (a few kHz up to a few MHz) are caused by the chopping technique used by variable speed drives, which generate major current spikes through the stray capacitance of circuits. Leakage current of a few tens or hundreds of mA can follow (common mode) and be detected by the RCD. Unlike the 50/60 Hz leakage currents for which the algebraic sum is zero, these HF currents are not synchronous over all three phases and their sum constitutes a non-negligible leakage current. RCDs type A and B are protected against these HF currents.

For combination of RCDs and VSDs using frequency conversion, it is necessary to simultaneously take into account a number of constrains.

    • Leakage current when energising


    • Continuous leakage current at 50/60Hz


    • Continuous HF leakage current


    • Special current waveforms for faults at the device output


  • Current with the DC component for faults on DC bus

What is recommendation for using of RCDs in TT, TN and IT systems?

TN System
In a 230/400 V TN system, the touch voltage Ud is 92 V.

Ud = 0.8 U0 / 2
Id = 0.8 U0 / (Rph + RPE)

Rph : Resistance of the phase conductor
RPE : Resistance of the PE conductor

This voltage is greater than the conventional touch voltage limit and represents a danger, i.e. the circuit must open. In general, given the level of the fault current Id, opening can be initiated by over-current detection devices. When the resistance values Rph or RPE are high or unknow, RCD protection is required.

TT System
In a 230/400 V TT system, the touch voltage Ud is 115 V.

Ud = U0 × Ra / (Ra + Rb)
Id = U0 / (Ra + Rb)

Ra : Resistance of the earth at consumer side
Rb : Resistance of the earth at power supplier side

This voltage is greater than the conventional touch voltage limit and represents a danger, i.e. the circuit must open. In the earth resistance in approximately 10 Ω, the fault current is approximately 11 A. In general, opening cannot be initiated by the over-current detection devices. Use of the RCD is therefore mandatory.

IT System
Even the high leakage capacitances of 1 µF, the leakage current for the first fault is less than 0.1 A. the result is a harmless touch voltage of approximately one volt. Disconnection is not necessary for the first fault. If the second fault occurs, the situation is that of the TN system.

Is there any rough relation between cut off current and rated current of a fuse?

At a prospective current of Ip(A), the cut off current Io(A) of a fuse-link of rated current of In(A) is equal to or less than the value given by the formula:

What are types of coordination and which standard applies?

Two types of coordination are recognized in IEC 60947-4-1; type 1 and type 2 (for full details of tests and acceptance criteria see IEC 60947-4-1).

Performance requirements Type 1 Type 2
The short circuit is successfully interrupted yes yes
Persons are not endangered yes yes
Conductors and terminals remain intact and undamaged yes yes
No damage to an insulating base which dislodges live parts yes yes
No damage to overload relay or other parts no yes (note 1)
No replacement of parts permitted during test (other than fuses) no yes
No change in overload relay tripping characteristics no yes
Starter insulation level satisfactory after test no yes

Note 1: easy separable welding of contacts permitted.

When must current limiting fuses (CLF) be implemented in the circuit protection?

If short circuit current (Peak) exceeds short circuit withstand capacity of the breaker the current limiting fuses shall be used in addition to the circuit breakers.

What do you recommend for settings of thermal overload current and short circuit current?

Set the thermal overload current (TOL) 1.05 times of normal operation current In. Short circuit current  (I>) shall be greater than starting current e.g. if motor’s LRC = 6×In, then set I> to 7×In.

Is it possible to install a RCD (Residual Current Device) in downstream of a VSD?

It is not permissible that the convertor in a VSD panel is connected up through an ELCB (ground fault circuit interrupter), DIN VDE 0160.

What are the criteria for selecting fuses for feeders?

For LV feeders fuses are selected as per AS/NZS 3000 clauses 2.4.2 and 2.4.3 to provide protection to the cable for both overload currents and short circuit currents. Note that fuses are not intended to protect the connected equipment from overload currents. They are only intended to protect the cable. Fuses shall be selected to coordinate with the load current and the cable current carrying capacity and must satisfy the following conditions.

    • Nominal load current Nominal fuse current Cable current carrying capacity


  • The current causing the fuse to definitely blow does not exceed 1.45 times the installed current carrying capacity of the cable.

AS/NZS 3000 clause states that the fusing current in conventional time for fuses is 1.6 × Nominal fuse current (from AS/NZS 2005 series). Therefore, the criteria above can be simplified to a single condition.

Nominal load current Nominal fuse current 0.9 × Cable current carrying capacity

Note that the installed cable current capacity shall be considered in above equation. The installed cable current capacity is less than maximum cable current capacity taking into account grouping and temperature factors.

Fuse selection for motor feeders

For motor feeders, fuses are selected to protect cables and motor starter equipment from short circuit only. Refer to AS/NZS 61459 Annex A for related table. Protect against overload currents that can potentially damage the cable is assumed to be included in the motor protection modules. Therefore, fuses do NOT have to be coordinated with cable current carrying capacity.

What are the criteria for selecting miniature circuit breakers?

For lighting cables and small power services, short circuit protection and overload protection for the cable shall be provided by MCBs. The MCBs will be equipped with earth leakage protection.

  • MCBs for single phase 240 V lighting and general power outlets (GPOs) shall be rated for 16 A.
  • MCBs for three phase 415 V welding sockets shall be rated for 63A (is applicable) or as required to match the socket rating.

What is recommended disconnection time for circuit breakers?

The total amount to be allowed to cover the following items

  • The fault current interrupting time of the circuit breaker
  • The overshoot of the relay
  • Errors
  • Final margin on completion of operation

depends on the operation speed of the circuit breaker and the relay performance. At one time 0.5 s was the normal grading margin. With faster modern circuit breakers and lower relay overshoot time 0.4 s is reasonable, while under the best possible condition 0.35 s may be feasible.

Disconnection time are stipulated in AS/NZS 3000 clause for a single phase to earth fault

  • 0.4 sec. for socket outlets (not exceeding 63 A) and portable equipment
  • 5 sec. for other circuits

What are the criteria for selecting overload protecting devices?

Based on AS/NZS 3000, section 2.5.3, 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)

In which

  • 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 (considering all de-rating factors)
  • I2: The current ensuring effective operation of the protective device and may be taken as equal to either
    • The operating current in conventional time for circuit breakers (1.45 In); or
    • The fusing current in conventional time for fuses (1.6 In for fuses in accordance with the AS/NZS 60269 series).

To satisfy equation 2.2, the nominal current In of a fuse should not exceed 90% of Iz (1.45/1.6 = 0.9) therefore

  • Ib ≤ In ≤ Iz     For circuit breakers
  • Ib ≤ In ≤ 0.9 × Iz     For HRC fuses

What are criteria for limiting of the harmful effects of a switchboard internal arcing fault?

Based on AS/NZS 3000, for Limitation of the harmful effects of a switchboard internal arcing fault, Protective devices shall be provided to limit, as far as practicable, the harmful effects of a switchboard internal arcing fault by automatic disconnection.

The arcing fault current between phases, or between phase and earth, is deemed to be in the range of 30% to 60% of the prospective short circuit current. Protection shall be initiated, i.e. pick up at a current less than 30% of the three-phase prospective fault level. To minimize damage to the switchboard, the interrupting time shall not exceed the value obtained from the following equation. The general damage limit is given by the following:

Clearing time

  • t = clearing time in seconds
  • lf = 30% of the prospective fault current
  • lr = current rating of the switchboard
  • ke = 250 constant, based on acceptable volume damage

Example. The maximum arcing fault clearing time at a customer’s 800 A-rated main switchboard with a prospective fault current at the switchboard of 16.67 kA.

Therefore, If = 30% of 16.67 kA = 5 kA.

Clearing time example
i.e. The protective device settings must be set to clear an arcing fault of 5 kA in less than 0.57 s.

Overcurrent (short circuit) protective devices should be set to as low an initiation current as possible while still maintaining the correct function of the installation, e.g. set higher than motor-starting currents. Earth fault protective devices shall have a maximum setting of 1200 A.

NOTE: The electricity distributor should be consulted for discrimination requirements between installation protective devices and the electricity distributor’s service protective devices. The curves and settings of service protective devices will be required.

Where arc detectors are used, immunity to extraneous light sources that may cause operation of the protection is necessary. Arc detectors do not obviate requirements for discrimination.

What is restriction on under-voltage protection devices to cope motor starting?

AS/NZS 3000, section 2.8.2, Protective devices having time-delay facilities should permit the starting of motors where the supply voltage exceeds 85% of rated voltage and continued operation where the voltage is within 10% of the rated voltage.

Is it possible to use directional over current relay instead of reverse power relay?

Reverse power relay is different in construction then directional over current relay. On directional over current relay, the directional element does not measure the magnitude of power. It sense only direction of power flow. However, In reverse power relays, the directional element measures magnitude and direction of power flow.

Non directional relays which are applied to two parallel power lines, disconnect both lines even fault occurs in one of them. What is the remedy?

If non-directional relays are applied to parallel feeders, any fault that might occur on any one line will, regardless of the relay setting used, isolated both lines and completely disconnect the power supply. With this type of configuration it is necessary to apply directional relays at the receiving end and to grade them with the non-directional relays at the sending end, to ensure correct discriminative operation of the relays during the faults. This is done by setting the directional relays with their directional element looking into the protected line, once giving them lower time and current setting than non-directional relays. The usual practise is to set directional relays to 50% of the normal full load of the protected circuit and 0.1 TMS, by care must be taken to ensure that their continuous thermal rating of twice rated current is not exceeded.

What is difference between over-current and over-load protection?

The term over-current includes both over-load and short circuit currents.

How can fault-loop impedance be calculated?

Based on AS/NZS 3000, section B4.5, the suitability of the particular overcurrent protective device depends on the value of the earth fault-loop impedance (Zs).

Zs = Zext + Zint


    • Zext: the impedance of the upstream circuit of the protection device.


    • Zint: the impedance of the downstream circuit of the protection device.


  • Zs: earth fault-loop impedance.

When an electrical installation is being designed, Zext may or may not be available (it will depend on the electricity distributor’s transformer and supply cables). If it is not available Zint may be determined by either of the following methods.

When the length and cross-sectional area of the supply conductors are not known, it may be assumed that there will always be 80% or more of the nominal phase voltage available at the position of the circuit protective device. Therefore, Zint should be not greater than 0.8 Zs. This may be expressed as follows:

Zint = 0.8 Uo/Ia


Uo = nominal phase voltage (230 V)
Ia = current causing automatic operation of the protective device, as follows:

  • Ia for circuit-breakers is the mean tripping current as follows:
    • Type B = 4 × rated current
    • Type C = 7.5 × rated current
    • Type D = 12.5 × rated current
  • Ia for fuses are approximate mean values from AS 60269.1.

What guidelines apply for SPD installation?

For must domestic single-phase suppliers in urban environment, a surge rating of Imax=40 kA per phase for an 8/20 µs impulse and a minimum working voltage of 275 V a.c. is suitable.

In the case of installation in exposed locations, e.g. high lightning area, long overhead service line, industrial and commercial premises, it may be prudent to install SPDs with a higher surge rating, typically 100 kA per phase for an 8/20 µs impulse.

Recommended surge ratings

Category SPD Location Imax Rating
A Long final sub circuits and electrical supply outlets 3-10 kA
B Major sub mains, short final sub circuits and load centres 10-40 kA
C1 Service entrance, other than below 40 kA
C2 Service entrance, building fed by long overhead service lines, or is a large industrial or commercial premises 40-100 kA
C3 Service entrance, building in the high lightning area, or fitted with a LPS 100 kA

A circuit breaker or fuse should be installed upstream of the SPD to protect the SPD. A 40 kA SPD may be protected by a 32 A HRC fuse or 20 A circuit breaker having a breaking capacity not less than the prospective short circuit current at the point of installation or by other means recommended by the manufacturer.

Typically for an SPD of surge rating 100 kA, a 63 A HRC fuses, with fault rating to suit the switchboard fault current is suitable and the wiring size should be 16 mm2. Where the electricity fuse supply is less than 63 A, it may be necessary to reduce the size of the SPD series fuse to avoid tripping the supply fuse in the event of an SPD failure or extreme over current transient disturbance.

Recommended backup fuse/CB rating for SPDs

Imax surge rating (kA) Recommended HRC fuse (A) Recommended CB (A)
25 20 10
40 32 20
100 63 40

Note that over current protection devices such as fuses and circuit breakers respond to heat or currents caused by the down-line-fault. These devices are too slow to react to transients.

SPDs (Surge Protection Devices) should be installed after the main switch by prior to any RCD devices (Residual Current Device). If and SPD is installed on the load side of an RCD, the RCD shall have a breaking capacity of not less than 3 kA. S type RCDs, in accordance with AS/NZS 61008-1 and 61009.1, are deemed to satisfy this requirement.

Conductors used to connect an SPD to bus the line via the overcurrent protective device, and to the main earthing or neutral conductor, should be not less than 6 mm2 and as short and direct as possible, with no loops. The total conductor length between two points of connection, including both active and earth/neutral, must be less than one meter, ideally 300 mm to 600 mm, overall. A connection to the neutral link should be as close as practicable to the MEN connection. If this is not possible secondary protection should be considered on sensitive circuits.

For MEN systems primary protector shall be installed between each phase conductor and neutral in the main switchboard. Since there is only one connection between neutral and earth at the main switchboard secondary protection should include all mode protection; that is protection both phase to neutral and neutral to earth. This may be achieved in a number of ways, typically by including an array of MOV arrestors (metal oxide) or a gas discharge tube (GDT) connected between neutral and earth.

The GDT is effectively an open circuit until it fires and electrically clamps the neutral and earth together. It then resets to its open circuit state. Once the GDT fired this device conducts at the voltage of typically 30 V. Therefore they cannot be used to protect power circuits between phase conductors and neutral but are entirely suitable for connection between neutral and earth. In this configuration they are extremely robust.

SPD Coordination
For proper coordination secondary series protection should be separated by at least 10m of distribution cable from the primary protection. If this is not possible shunt SPDs must be employed for secondary protection.

Is it required to protect circuits feeding electrical consumers located in hazardous area with earth leakage protection?

Some companies specify that all electrical power circuits feeding hazardous area equipment shall be fitted with earth leakage protection. However, In Australian standards it is mandatory just for heating equipment and recommended for other equipment.

Which protections are applicable for batteries?

As per AS/NZS 4509.2, the output conductors at the battery shall be protected against overcurrent, by high rupturing capacity (HRC) fuses or DC rated current breakers, as follow

    • Where the battery bank is electrically flouting (i.e. neither side of the battery is earthed), protection shall be provided in both positive and negative battery leads.


  • Where one side of the battery bank is earthed, protection shall be provided in the unearthed battery lead.

The protection should be mounted as close as practicable to the battery terminals (keeping battery leads as short as possible) while offering no possibility for spark ignition of any hydrogen emitted from the batteries during charging.

What is galvanic isolation?

Galvanic isolation is an arrangement within an apparatus which permits the transfer of signal or power between two circuits without any direct electrical connection between the two. Galvanic isolation frequently utilizes either magnetic (transformer or relay) or opto-couped elements.

What are protection requirements for resistance heating devices installed in hazardous area?

AS/NZS 60079.14, section 7.4

In addition to over current protection, and in order to limit the heating effect due to abnormal earth-fault and earth-leakage currents, the following protection additional shall be installed.

  • In a TT or TN type system, a residual current device (RCD) with a rated residual operating current not exceeding 100mA shall be used. Preference should be given to RCDs with a rated residual operating current of 30 mA.
  • In an IT system, an insulating monitoring device shall be used to disconnect the supply whenever the insulation resistance is not greater than 50Ω per volt of rated voltage.

AS/NZS 60079.14, section 8.1

Emergency switch off should consider isolation of all circuit power supply conductor including the neutral.

What is earth leakage relay?

Earth leakage relay are used in particular where ground (earth) fault detection is required or the fault loop impedance is of such a level that the over-current device does not achieve automatic disconnection within the times prescribed in the wiring rules AS 3000.

Is it permitted to use RCDs to protect emergency supply?

In circumstances where automatic disconnection of the electrical equipment may introduce a safety risk which is more dangerous than that arising from the risk of ignition alone, a warning device (or devices) may be used as an alternative to automatic disconnection provided that the operation of warning device (or devices) is immediately apparent so that prompt remedial action will be taken.

What if short circuit current (peak) exceeds short circuit withstand capacity of the breaker in circuit?

If short circuit current (peak) exceeds short circuit withstand capacity of the breaker, the current limiting fuses (CLF) shall be used in addition to the circuit breakers.

What is zero-sequence current transformer?

Core balance current transformer is also called as zero-sequence current transformer (ZSCT).

What is negative-sequence current/voltage?

Negative sequence is one of three quantities used in the symmetrical component analysis of three-phase power systems. Symmetrical components are used to calculate unbalanced condition on a three-phase system using only a single-phase calculation. This greatly simplifies the process of calculating fault quantities for phase-to-phase, phase-to-ground and phase-to-phase-to-ground faults on power systems. Symmetrical components consist of positive, negative and zero-sequence quantities. Basically, positive-sequence quantities are those present during balanced, three-phase conditions. Positive-sequence quantities makeup the normal voltages and currents observed on power systems during typical steady-state conditions. Negative sequence quantities are most commonly associated with ground being involved in an unbalanced condition. Negative and zero sequence quantities are usually only present in substantial levels during unbalanced, faulted conditions on a power system.

Since negative and zero sequence quantities are only present in relatively large values for faulted conditions, they are often used to determine that a faulted condition exists on a power system. Negative-sequence can be used to detect phase-to-phase, phase-to-ground and phase-to-phase-to-ground faults. Zero-sequence can be used to detect phase to ground and phase-to-phase-to-ground fault.

Negative-sequence impedance is defined as negative-sequence voltage divided by negative-sequence current. For a forward fault, the value of the negative-sequence impedance is always negative, and for the reverse faults, the negative sequence impedance is always positive.

There is no circuit breaker considered between the generator and the LV/ HV transformer in some plants. Is it acceptable design?

A generator and a transformer may form a unit. In this case it is not necessary to put a circuit breaker between them. Put the circuit breaker at the output side of the transformer (high voltage side). One of the benefits of forming a generator-transformer unit is that the leakage induction of transformer will considerably reduce the generator short circuit current.

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