⚡ SANS 10142 Testing Principles Series

Test 6: Prospective Short-Circuit Current (PSC) — Proving the System Can Handle Fault Energy

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Introduction

Up to now, we’ve confirmed:

• Test 1: Bonding is correct
• Test 2: Earth continuity is acceptable
• Test 3: Circuits are complete
• Test 4 & 5: Protection will operate under fault conditions

Now we move to a different but equally critical question:

👉 Can the system safely handle the fault current when it happens?

Because when a short circuit occurs…

👉 The current can rise to extremely high levels in milliseconds.

According to SANS 10142-1 (Clause 8.6 – Testing):

👉 The prospective short-circuit current (PSC) must be measured or determined to ensure that protective devices can safely interrupt the fault current.

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🧠 1. Foundation (Understanding): What Are We Actually Testing?

⚡ What Is PSC?

PSC is the maximum current that can flow during a short circuit.

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🔑 Types of Faults Considered

👉 Phase to Neutral (L–N)
👉 Phase to Phase (L–L) (where applicable)

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🔍 What This Test Proves

👉 That the fault current will NOT exceed the breaking capacity of the protection device

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⚠️ Why This Matters

If PSC is too high:

🚨 Breaker may fail to interrupt
🚨 Equipment damage occurs
🚨 Arc flash risk increases
🚨 Fire or explosion risk increases

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🛠️ 2. Application (Doing): How Must the Test Be Done?

⚠️ IMPORTANT

👉 This test is conducted under live conditions
👉 Using a PSC / loop impedance tester

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🔧 Measurement Method

PSC is typically determined by:

👉 Measuring loop impedance and calculating PSC
OR
👉 Using a tester that directly displays PSC

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⚡ Typical Connection

For single-phase systems:

👉 Live → L
👉 Neutral → N

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📊 Relationship Between PSC and Impedance

👉 PSC is inversely related to impedance:

• Low impedance → High fault current
• High impedance → Lower fault current

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🎯 PASS REQUIREMENT

👉 Measured PSC must be:

🔥 LESS than the breaking capacity of the protective device

Example:

• Breaker rated at 6 kA
  👉 PSC must be below 6 kA

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⚡ 3. Mastery (Owning Responsibility): What Are the Limitations?

This is where professionals stand out.

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🔍 The Professional Must Ask:

👉 What is the breaker breaking capacity?
👉 What is the supply strength?
👉 Is this near a transformer?
👉 Are there parallel supply paths?
👉 Has the installation been extended?
👉 Is the measured PSC the worst-case scenario?

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⚠️ Critical Understanding

PSC is highest:

👉 Close to the transformer
👉 At the origin of the installation

And decreases further along the circuit.

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⚠️ Common Mistakes

• Not measuring PSC at origin
• Ignoring breaker breaking capacity
• Confusing PSC with load current
• Assuming standard values
• Not considering supply variations

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💡 Final Thought

Test 6 is about energy control.

Because during a fault:

👉 It’s not just about whether it trips…
👉 It’s about whether it survives the interruption.

At TDMI Training, we emphasise:

👉 Protection must not only operate
👉 It must withstand the fault safely

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✅ The Standard

• Measure or calculate PSC
• Compare to breaker rating
• Verify equipment suitability
• Understand system conditions

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👉 Because when a short circuit happens…
the system must contain it instantly.