Prospective Fault Current
Which test instrument is used for measuring prospective fault current?
1. A Prospective Fault Current tester OR 2. the prospective fault current range of an EFLI tester
Prospective Fault Current must be determined for which TWO situations?
1. Short Circuit currents 2. Earth Fault currents
Having obtained a value of PFC, what must the tester do?
Compare the value obtained with the breaking capacity of the protective devices.
Between which conductors must prospective fault current be measured?
Line and Earth Line and Neutral AND for three phase installations, Line and Line
Which THREE methods can be used to determine Prospective Fault Current?
Measurement (Testing) Calculation Enquiry (to the DNO)
The breaking capacity of protective devices is split into how many different categories?
TWO Icn (the rated short-circuit capacity) AND Ics (the service short-circuit capacity)
Which value of PFC is to be recorded?
The HIGHEST value of either the short circuit current (between live conductors), or earth fault current (between line conductor and earth)
Which of the two breaking capacity categories is marked on the protective device?
The Icn rating i.e. the maximum PFC the device can break, although it may not work afterwards.
How is a comparison between the PFC of the installation and the breaking capacity of the protective devices made?
The breaking capacity of the protective device must be at least equal to, or greater than, the PFC value.
What is the difference between Icn and Ics categories of breaking capacity?
The condition of the protective device after manufacturers testing
Except in London, and some other major city centres, the maximum PFC for single phase 230V supplies up to 100A is unlikely to exceed which figure?
16kA
For domestic properties what is the suggested value that the PFC at the origin of an installation is unlikely to exceed?
6kA
Where, in an installation, MUST Prospective Fault Current be determined?
Anywhere that fuses and circuit breakers are fitted i.e. 1. The origin of the installation (main Distribution Board) 2. Any other DB's
If the protective devices at a remote distribution board have the same breaking capacity as the devices at the origin of the installation, why does the PFC not need to be checked at the remote location?
Because as you get further away from the origin of the installation the magnitude (size) of the PFC will decrease
If your test instrument isn't capable of testing between Line and Line (because of the 400V present) how can the PFC between lines be estimated?
By multiplying the single phase measurement by 2
Under which circumstances would the breaking capacity of protective devices at a remote distribution board not need to be checked?
If they have the same breaking capacity as the protective devices fitted at the origin of the installation
What condition would an Ics rated device be in after manufacturers testing?
The protective device will break it's rated PFC and will still operate normally afterwards
What condition would an Icn rated device be in after manufacturers testing?
The protective device will break it's rated PFC but may be damaged in the process and may no longer work afterwards
