Loop impedance and PSCC: the numbers a test must show
SANS 10142-1 8.6.5 and 8.4, in plain language. No single “pass” ohm value exists: it depends on the device.
The rule in one line
At the main switch, the earth loop impedance must be low enough that an earth fault pushes at least 3 × the rated current of the protective device through it, so the device disconnects automatically. That rule holds for devices up to 63 A. Above 63 A the disconnection time must not exceed 5 s. (8.6.5.1)
The book also gives the arithmetic: Z ≤ U0 / Ia, where Ia is the current that trips the device within the time allowed and U0 is the nominal line-to-earth voltage. (8.6.5.1 NOTE 3)
Maximum earth loop impedance
Pick the device and its instantaneous tripping multiple. Manufacturer data wins; the book's NOTE 1 says the multiple ranges from 3× to 20× and that 10× is the typical assumption in the field when product data is missing.
A
V
Zs must not exceed—
Fault current Ia—
Rule that applies—
—
Where you measure it
At the main switch, not at a socket. 8.6.5.2 says the earth fault loop impedance is measured there to prove 8.6.5.1.
The neutral loop too, on every low-voltage system that has a neutral. Same instrument, both clamps onto the neutral conductor. Where possible switch the main switch off and disconnect the main neutral so parallel paths don't flatter the reading. It should read lower than the earth loop, or at least substantially the same, and never higher. (8.6.5.3, 8.6.5.4)
Record what the instrument gave you, not a number from a table. The pass line is calculated from the actual device in the actual board.
When it cannot comply
8.6.5.5 allows an alternative: an earth fault detection and disconnecting device at the supply, set to operate on the current the loop impedance gives, so that touch voltages above 25 V are not exposed longer than the times in Table 8.3.
TN or IT · 25–50 V1,6 s
TN or IT · 50–120 V0,8 s
TN or IT · 120–230 V0,4 s
TT · 25–50 V0,5 s
TT · 50–120 V0,3 s
TT · 120–230 V0,2 s
Table 8.3, first three touch-voltage bands. The table goes on to 400 V and above (0,2 / 0,1 s for TN or IT; 0,07 / 0,04 s for TT).
PSCC: a different test, a different number
8.4.1: determine the prospective short-circuit current at the point of supply or control for the board — and also at the point of control if that sits away from the point of supply.
8.4.2: for systems rated up to 250 V to earth, measure it at the point of control with a fault current meter. Before you connect anything, check the meter is rated for the job, especially where the main switch is over 100 A or you expect more than 6 kA.
Three-phase: don't measure it unless the meter is made for it. In a balanced system the three-phase value is about 1,73 × the single-phase reading.
Why it matters: switchgear must be rated to withstand the prospective short-circuit current at that point (6.6.1.14). The kA figure printed on a breaker is not decoration.
The book sets no pass value for PSCC. It sets a measurement duty, and the equipment you install has to survive whatever you measured.
VOLT does this maths on the board you are standing at
Enter the device and the app works out the maximum loop impedance, checks your reading against it, and carries it onto the CoC test report and the PDF. Offline, on your phone.
Values from the SANS 10142-1 Edition 3.3 public-enquiry draft (clauses 8.4 and 8.6). Your licensed copy of the book wins over this page. VOLT is a tool, not the standard: check the clause in your own book before you sign anything.