Short Circuit Current Rating and Fault Level Current

The Correct Method to be used When Testing an Electrical Installation for Earth Loop Impedance: Accurately Measuring the Value of Zs at a Remote End of a Circuit. © By: Dave Rongey

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Guide to Testing an Electrical Installation for Earth-Loop-Impedance
Electric Question: Whilst writing, I would appreciate your views on the argument that reducing the value of Ze ie. by say the use of a cpc in parallel with an SWA, will have the reactive result of potentially presenting a higher fault level current (KA) – of course the cpc can always be disconnected if Ze is to high, or of course the prospective short circuit current rating of protective devices could always be increased – given the option(s), what course of action would you take?

Background: Bob, a Contractor from Middlesex, England.

Dave’s Reply:
Thanks for your electrical question Bob. The following information will assist you with Earth Loop Impedance:

Testing for Ze, Zdb and Zs

THE CORRECT METHOD TO BE USED WHEN TESTING AN ELECTRICAL INSTALLATION FOR EARTH-LOOP-IMPEDANCE

• Accurately Measuring the Value of Zs at a Remote End of a Circuit
• In order to make an accurate measurement of Zs at a remote end of a circuit, we must first understand what we are trying to measure. As in any test for earth-loop-impedance, the value we are attempting to evaluate is the impedance, in other words, the ‘total opposition to an a.c. current’.
  • In our case, this ‘a.c. current’ is our earth-fault current.
  • For this measurement to be carried out correctly we need to ensure that only the phase and cpc conductors are involved in the measurement process.
• It follows then, that any additional fault paths in an electrical installation will have a detrimental effect on the measured Zs values of an installation.
  • Using the formula for parallel resistor will prove this. (R1xR2 divided by R1+R2 as will be demonstrated later).
• For our measurements we give the conductors the titles of ‘ R1’ for the phase and ‘R2’ is the cpc.
• Starting with Ze, we will investigate the procedures involved in accurately recording the earth-loop-impedance data obtained from the intake position and move onto the final sub-circuits of an electrical installation.
  Ze
• In the first instance the installation must be taken off-line so that the equipotential bonding may be disconnected from the earth conductor and the means of earthing.
  • These disconnections remove any parallel paths that water and gas service pipes may otherwise offer.
• If we assume that a ‘three wire’ test instrument is to be used for testing and measurement we connect as follows:
  • Start with the ‘Green’ clip. Connect this clip or probe to the means of earthing.
  • This could be the rod of a TT system, the lead armored of a TN-S or the termination bar bolted to the neutral return conductor of a TN-C-S system.
  • The ‘Black’ or neutral lead is connected to the neutral supply (along with the ‘Green’ in a TN-C-S system)
  • Finally connect the ‘Red’ probe to the phase supply.
• The measurement of Ze is now carried out and recorded.
  • The instrument is re-set (if allowed for by the manufacturer) and a PSCC value is also recorded.
  • If there is no facility on the instrument for a PSCC test then divide the voltage by the ohms value to give you the ‘earth-fault’ PSCC in amps.
• Please note: The correct, and therefore safe, removal of these clip and probe connections are carried out in the following order.
  • Phase off; neutral off and then the ‘Green’.
  • Remember that the ‘Green’ cable is NOT an earthing cable but a switched LIVE!
• A further test is then carried out with the instruments ‘Green’ and ‘Black’ leads connected to the neutral supply conductor with the ‘Red’ lead connected to the phase.
  • On testing, the reading given in ohms is divided into the voltage giving a ‘Short-circuit’ fault current.
  • The greater of the ‘earth-fault-current’ and the ‘short-circuit’ is the value that is to be recorded as the systems PSCC current.
• Now we re-connect the equipotential bonding and (subject to satisfactory values being recorded) the system re-energized.
  Zdb
• With the equipotential bonding conductors re-connected we can make a Zdb test at the distribution board.
  • As before the leads from the test instruments are connected in the correct order- green – black – red.
  • A test is carried out and the value recorded. If there is any difference between the recorded value of Ze and the recorded value Zdb then this difference is the value of the parallel paths associated with the equipotential conductors and this value must then be added to all subsequent measured values of Zs.
  • ((R1+R2)x F) to be added to Zdb to calculate Zs
• R1 and R2 values are found in Table G1 of IEE Guidance Note 3 and the ‘Factor’ by which the given values are to be multiplied before they can be used to calculate Zs by can be found in table G2.
  • These values are vital when designing cable runs for circuits.
• When the cable has been selected for the required loading of the circuit and has been found to satisfy the volt-drop requirements, (Click here to go to the volt drop section) the next thing to do is verify that the maximum Zs rating of the protective device is not exceeded.
• If the cable is a 2.5mm twin & 1.5mm cpc then;
  • Multiply the quoted values of R1 +R2 (may be 0.01951 m ohms) from G1 by the length of run (try 20m).
• 0.01951 x 20 = 0.39ohm
  • from G2 we are given a factor of 1.20.
  • 0.39 x 1.2 = 0.46 ohm
• Then add the Ze of maybe 0.09 ohm to give a calculated Zs of 0.55 ohm.
  • If this value is less than the maximum value of the protective device then the circuit will comply with the regulations BS 7671:1992.

• The Following links will assist you with your electrical question:

• Inspection, Testing and Certification of Electrical Installation

• For more information about Grounding
  Electrical Grounding