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Prospective fault current: a three-phase scenario1.6 kA between L1 and neutral. What do you record?
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The question at the end: 1.6 kA between L1 and neutral. What do you record? The answer is in the notes below.
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Prospective fault current (Ipf) is the higher of the prospective earth fault current (PEFC) and the prospective short-circuit current (PSCC).
What is this test for?
- aChecking the distribution circuits are selective
- bChecking the protective devices can safely clear a fault within the installation
- cChecking the DNO's service fuse disconnects within 5 seconds
- dChecking the main switch can switch short-circuit currents
- c is out: the DNO's service fuse belongs to the DNO. It's outside the scope of BS 7671. Any disconnection rules for it come from the supply regulations, not ours.
- d is out: the main switch is an isolator. Nobody switches it off during a short circuit. By the time you'd reach it, the protective device has already operated.
- a is arguable. "Discrimination" is now called selectivity: the device nearest the fault operates first. For example, a heater fault on a DB2 circuit should trip that circuit in DB2, not the DB2 feed in DB1, which would kill every circuit in DB2. The test does relate to this, but b is better.
- b is the best answer, and it's the safety answer. Each device has a breaking capacity, which is the most fault current it can clear safely.
Take a B20 circuit breaker. In a small rectangle under the rating it's marked 10000. That's its Icn in amps. If a fault current is above that, the breaker may not clear it safely. It could explode, catch fire or weld its contacts.
A common mix-up is to call the "maximum before safety is lost" the Ics. That's actually Icn, the rated short-circuit capacity printed on the breaker (10 kA here). The device can break it safely but might not be fit to use afterwards. Ics is the service short-circuit capacity: the fault current it can break and still stay in service. Ics is equal to or lower than Icn.
ESQCR is the Electricity Safety, Quality and Continuity Regulations 2002, which cover the DNO's side.
If you find yourself thinking "it could be this or that", it is usually a sign you need more knowledge of the subject. Until then, the safety answer is the better guess.
GN3 · 2.6.16 prospective fault current · p.88 GN3 · Icn and Ics short-circuit capacities Read 2.6.14, 2.6.15 and 2.6.16 together and summarise them in your own words
The highest reading was 1.6 kA, between L1 and neutral. What goes on the certificate?
- a3.2 kA
- b1.6 kA
- c1600 kA
- d3200 kA
Answer a: 3.2 kA. The EIC's nature of supply parameters box has Ipf, just above Ze. On three-phase, the worst short circuit is between lines. But many testers would be damaged by 400 V across them. So, as the book says, test L1–N, L2–N and L3–N (230 V each), take the highest and double it.
"1600 kA" is 1600 × 1000 A, which is 1,600,000 A. It's like saying "sixteen hundred thousand pounds". Don't be lured by options c and d.
GN3 · Section 5 · EIC · nature of supply parameters (Ipf, above Ze) · example EIC pp.147–149 GN3 · 2.6.16 · three-phase: measure L–N, double the highest · p.88 Highlight this paragraph
What happens to prospective fault current the further into the installation you measure?
- aIt rises, because resistance falls
- bIt falls, because resistance falls
- cIt rises, because resistance rises
- dIt falls, because resistance rises
Answer d. It's not in the book, it's just Ohm's law. More cable means more resistance, and more resistance means less current. So PFC is highest at the origin, which is why you measure it there.
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