5.4.4 Provision of PME terminal

There is no maximum limit of loop impedance for a PME supply defined in either the current edition of BS7671 'Requirements for Electrical Installations' or Electricity Association Engineering Recommendation P23/1.

The values of loop impedance in Electricity Association Engineering P23/1 are indicative values for the majority of installations whether they use PME or other forms of earthing. They are not maximum values permitted for provision of PME.

Engineering Recommendation P23/1 'Customer's Earth Fault Protection for Compliance with the IEE Wiring Regulations for Electrical Installations.

This is guidance for electrical installation designers who need to use the loop impedance to ensure that sub circuit fuses clear within the times specified in the current edition of BS7671 'Requirements for Electrical Installations'. (ie 0.4 seconds for portable equipment or 5 seconds for stationary equipment). This requirement first appeared in the 15th edition of the IEE Wiring Regulation issued in 1981.

To meet this requirement, the installation designer needs to know the loop impedance at the cut-out of the property resulting from the length and size of Distribution Network Operator's LV network.

For a building with an existing service this value can be readily obtained by carrying out a loop impedance test. The value obtained must then be added to loop impedance of the internal wiring to determing whether the proposed fuse clearance times are satisfactory.

However, at the design stage of a new building there is no existing cut-out to test but the installation designer still needs to establish a loop impedance figure to base his design on. As he cannot carry our a loop impedance measurement at this stage he should normally ask the Supply Authority to provide the loop impedance of the proposed LV network at each property to be connected.

In order to prevent Distribution Network Operators being inundated with this type of enquiry, the Electricity Association produced Engineering Recommendation P23/1.

P23/1 providesa typical maximum value of LV netowrk loop impedance (eg 0.35 ohm for supplies up to 10 amps) with the caveat:

'Higher values could applyy to consumers supplied from small capacity pole transformers and/or long lengths of low voltage overhead lines.'

These values and the caveats on their use are reproduced in Central Networks 'Notes of Guidance to Electrical Contractors on Earthing and the Characteristics of Supply'.

Building installation designers will normally use the above figures to design circuit protection.

Central Networks' standard LV network design procedure will not normally result in the values being exceeded on Greenfield sites.

However in-fill developments and service alterations may results in higher values. In these cases the building installation designer must be advised of the actual values.