⚡ SANS 10142 Testing Principles Series

Test 4: Earth Fault Loop Impedance (Zs)

Test 5: Neutral Loop Impedance (Zn)

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Introduction

Up to this point, we have already confirmed the fundamentals:

• Test 1: Metal parts are bonded correctly
• Test 2: Earth continuity is within acceptable limits
• Test 3: Circuits are complete and intact

Now we move to the most critical question in the entire installation:

👉 Will the protection device operate fast enough under fault conditions?

This is where loop impedance testing becomes essential.

According to SANS 10142-1 (Clause 8.6.5):

👉 The impedance of the fault loop must be measured to ensure that sufficient fault current will flow to operate the protective device within the required disconnection time.

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

⚡ Earth Fault Loop (Zs)

This represents the total impedance of the path when a live conductor faults to earth.

👉 The path includes:

• Phase conductor (L)
• Fault point
• Earth conductor (E)
• Return path through the supply transformer

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⚡ Neutral Loop (Zn)

This represents the impedance of the phase-to-neutral loop.

👉 The path includes:

• Phase (L)
• Neutral (N)
• Return to source

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🔑 What These Tests Prove

👉 That sufficient fault current will flow
👉 That the breaker will trip
👉 That disconnection happens within safe time

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🖼️ Understanding the Loop Path (Illustration)

🔍 What This Illustration Shows:

• The complete fault loop path from DB → fault → earth → transformer → back to supply
• How current flows during a fault condition
• That loop impedance is not just one conductor—it is the entire system path

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

⚠️ IMPORTANT

These tests are done under live conditions using a loop impedance tester.

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⚡ Earth Fault Loop Test (Zs)

🔌 Correct Connections:

• Live → L
• Neutral → N
• Earth → E

👉 This measures the full earth fault loop path

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⚡ Neutral Loop Test (Zn)

🔌 Correct Connections:

• Live → L
• Neutral → N
• Earth → N

👉 This measures the phase-neutral loop impedance

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⚠️ Key Principle

The tester injects a test current and measures:

👉 The total impedance of the loop path

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📊 Determining Maximum Allowable Resistance

This is where correct interpretation becomes critical.

$R_{max} = \frac{V}{3I}$

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🔍 What This Means

• Rmax = Maximum allowable loop resistance
• V = Supply voltage
• I = Current required to trip protection

👉 The factor of 3 introduces a more conservative safety margin (aligned with upcoming practice)

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⚡ Practical Insight

• Lower impedance → higher fault current
• Higher fault current → faster disconnection
• Faster disconnection → safer installation

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🎯 PASS REQUIREMENT

👉 Measured loop impedance must be less than or equal to Rmax

AND

👉 Must ensure compliance with required disconnection times

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

This is where professionals separate themselves.

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🔍 The Professional Must Ask:

👉 What is the breaker curve (B, C, D)?
👉 What current is required for instantaneous trip?
👉 What disconnection time applies?
👉 Is the supply stable?
👉 Are there parallel paths influencing readings?
👉 Is the installation modified or extended?

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⚠️ Real Risk

If loop impedance is too high:

🚨 Fault current too low
🚨 Breaker may not trip in time
🚨 Dangerous touch voltage remains

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⚠️ Common Mistakes

• Incorrect tester connections
• Confusing Zs and Zn
• Ignoring calculation (Rmax)
• Not relating readings to protection
• Testing incorrectly under load conditions
• Assuming “reading looks fine”

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💡 Final Thought

Test 4 & 5 are where everything comes together.

Because now we are not testing wiring…

👉 We are testing protection under real fault conditions

At TDMI Training, this is where the standard is clear:

👉 If you don’t understand loop impedance…
👉 You shouldn’t be signing a COC

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✅ The Standard

• Connect correctly
• Measure accurately
• Calculate Rmax
• Interpret the result
• Confirm protection will operate

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👉 Because when a fault happens…
the system must respond instantly.