Earth loop impedance and prospective short circuit (PSC) testing methods

March 13th, 2009, Published in Articles: Vector


Why earth fault loop impedance test is necessary

Earth fault loop impedance is the path followed by fault current when a low impedance fault occurs between the phase conductor and earth, i.e. “earth fault loop”. Fault current is driven round the loop by the supply voltage. The higher the impedance, the lower the fault current will be and the longer it will take for the circuit protection to operate.

To make sure the protection operates fast enough, the loop impedance must be low. Every circuit must be tested to make sure that the actual loop impedance does not exceed that specified for the protective device concerned. It is recommended that the Ze test be done first. This test, done at the distribution board, gives the loop impedance of the circuit, excluding the installation. The Zs test, which includes the circuit tested in the Ze test as well as including the installation resistance, must be done next.

In most homes, basic shock protection is done by coordinating an earthing circuit with automatic switches in the indoor wiring circuits. This quickly cuts off supply to an earthing circuit where a fault occurs and touch voltage exceeds an acceptable limit. Proper protection against electric shock hazards is given when the TT wiring system complies with: Ra x Ia <50, where Ra is the sum of the resistances of earth bars and protective conductors and Ia is the maximum current of the protection system. Ra multiplied by Ia should not be more than 50 V, i.e. the maximum voltage one can touch will not exceed 50 V in the event of an earth fault.


Earth fault loop impedance testing

The value of the earth fault loop impedance is the sum of transformer coil winding resistance, phase conductor (L1) resistance and the protective conductor (PE) resistance as well as the source earth resistance and installation resistance.

The Ze earth fault loop impedance measurement is made on the supply side of the distribution board and the main means of earthing, with the main switch open and all circuits isolated. The means of earthing will be isolated from the installation’s earthing system (earth rods) bonding during the test. The Ze measurement will confirm the earth fault loop impedance as the sum of the resistances of the transformer coil winding, phase conductor or supply side and protective conductor resistance, but not the installation earth resistance.

The Zs earth fault loop impedance should be tested at the furthest point of each circuit. In most cases the circuit breaker needs to be bridged out. The total earth fault loop impedance can be measured by plugging a loop tester into a socket outlet, or in some cases with an external earth probe. The value of the earth fault loop impedance is the sum of the resistances of the transformer coil winding , phase conductor (L1) and protective conductor (PE) as well as source earth and installation earth resistance.

When using an external earth probe, the earth fault loop impedance can be measured by touching an external probe directly to an earth bar, collector and connection point of an earth bar. The same measurement can be done by touching the earth probe to exposed, conductive parts of equipment in the circuits and exposed metal parts.

Anyone on site must avoid shock hazard while establishing contact and while doing the test. When buying a loop tester ask for distribution board test leads so that Ze and Zs measurements can be done.

Prospective short circuit current (PSC) testing

The prospective short circuit or fault current at any point in an electrical installation is the current that would flow in the circuit if no circuit protection operated and a complete (very low impedance) short circuit occurred. The value of this fault current is determined by the supply voltage and the impedance of the path taken by the fault current. Measurement of PSC can be used to check that protective devices within the system will operate within safety limits and as per the safe design of the installation. PSC is normally measured between the phase and neutral at the DB or at a socket outlet.

Contact Wayne Ternant, Major Tech, Tel 011 822-1551,

  • brian

    Why would apollo 2000 require a martindale polarity and loop tester to connect electric cooker I understand the to verify correct polarity but I would imagine loop impedance is beyond the average cooker installer as these people have no college training and experience in the use of would anybody disagree

  • Francis Nagel

    If a ups is installed on a DB as an alternative power supply what is the procedure during psc testing. Can a UPS be tested separately during such a test?

    • Kuho

      Try not to test but use calculations!!

      The S/C capability of a (true-online static) UPS would typically be way lower than the mains.You should be able to get it from the tech data sheets.

      However the UPS could be in internal bypass when a fault occurs so you would need to consider not just the UPS S/C.

      BUT also consider that some UPSs are transformer-based. The transformer may be on the input, on the bypass or on the output, or even on both input and output. If it has an output and/or bypass transformer its likely a Dyn in which case you must use the UPS transformer Ze and not that of mains… unless you also have an external bypass in which case you must again consider both mains and UPS s/c.

      Also be aware that some UPSs automatically go into internal bypass when feeding a short

  • Yasir

    Can any one tell me if i got a higher loop impedance value of approx 5 ohms for MCCB 63 Ampere breaker with having distance of 300 meters from Main source breaker with local Earth bus bar system is used. Type of cable used is 3 x 50 mm2 with 35 mm2 local earthing cable.My question is How to minimize this value upto 0.27 ohms or by use of RCBO or other?
    Expert opinion are required thanks.

  • zee

    why must you never connect a normal platform earth and an I.S. earth together

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