Selectivity between protective devices means:

only the device nearest the fault operates, leaving other circuits unaffected

all upstream devices operate together, so the fault is cleared as fast as possible

the main switch always operates first, isolating the whole installation

no device operates until the fault current falls to a safe level, protecting the supply

Back-up protection occurs when:

an upstream device clears a fault if the closer device fails to operate

two devices always operate simultaneously

the downstream device protects the upstream device

the supply is disconnected before any device operates

Time-current characteristics of protective devices are important for:

understanding how devices respond at different fault-current levels

confirming that the rated current is at least the design current

calculating the nominal supply voltage at the origin of the circuit

estimating the connected lighting load of the building being designed

Free mock exam

Protection Coordination — Level 3 Systems Design Practice

Free topic-focused mock exam: 30 random questions from a pool of 44 on Protection Coordination, drawn from the wider Level 3 Systems Design bank. 35-minute timer, 70% to pass, worked explanation on every question. Use this when you've taken the full mock once and want to drill the topics you got wrong.

Questions: 30
Time limit: 35 min
Pass mark: 70% · 21/30
Question bank: 44

Free, no sign-up. Retake as many times as you like — different questions each time.

Sample questions

01Selectivity between protective devices means:
- Ano device operates until the fault current falls to a safe level, protecting the supply
- Bonly the device nearest the fault operates, leaving other circuits unaffected
- Cthe main switch always operates first, isolating the whole installation
- Dall upstream devices operate together, so the fault is cleared as fast as possible
Why: Discrimination ensures the protective device nearest the fault operates first, maintaining supply elsewhere.
02Back-up protection occurs when:
- Atwo devices always operate simultaneously
- Bthe supply is disconnected before any device operates
- Cthe downstream device protects the upstream device
- Dan upstream device clears a fault if the closer device fails to operate
Why: Back-up protection provides secondary protection if the primary device fails to operate.
03Time-current characteristics of protective devices are important for:
- Aunderstanding how devices respond at different fault-current levels
- Bconfirming that the rated current is at least the design current
- Cestimating the connected lighting load of the building being designed
- Dcalculating the nominal supply voltage at the origin of the circuit
Why: Time-current curves show the operating time at different current levels, which is essential for coordination.

Common questions

Is the Protection Coordination mock exam free?

Yes, it is free with no sign-up needed. Questions are pulled at random from a substantial bank, with a worked explanation on every question after you submit.

Can I retake the mock exam?

Yes. Every attempt picks a different random selection and reshuffles the options, so each retake gives you genuinely new questions. No limit on retakes.

Does this count as a qualification?

No. This is independent practice material and does not award a qualification, card or certificate. Where a subject has a formal qualification or card, you gain it through an approved provider.