⚡ SANS 10142 Testing Principles Series

Test 9: Voltage at No Load — Proving the Supply Is Correct Before Use

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Introduction

Up to now, we’ve verified:

• Test 1–3: Installation integrity
• Test 4 & 5: Protection operation
• Test 6: Fault energy (PSC)
• Test 7: Supply neutral condition
• Test 8: Insulation integrity

Now we confirm something fundamental:

👉 Is the supply voltage correct at the point of control before any load is applied?

Because even a perfect installation…

👉 Is unsafe if the voltage is wrong.

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

This test checks:

👉 The actual supply voltage under no-load conditions
👉 At the point of control (Mains)

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⚡ What Is “No Load”?

👉 The installation is energised
👉 But no current is being drawn

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🔑 What This Test Proves

👉 Supply voltage is within acceptable limits
👉 Installation is safe to energise
👉 No abnormal supply condition exists

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

⚠️ IMPORTANT

👉 Test is done at the Point of Control (main switch / mains)
👉 Under no-load conditions

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⚡ Single-Phase (1Φ)

👉 Measure between:

L – N ONLY

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⚡ Three-Phase (3Φ)

👉 Measure between:

• L1 – N
• L2 – N
• L3 – N

👉 All measurements done at the Point of Control (Mains)

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⚡ Instrument

👉 Reliable voltmeter
👉 True RMS preferred

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📊 PASS REQUIREMENT

⚡ Nominal Supply (SA)

👉 230 V (Single Phase)

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🎯 Acceptable Tolerance

👉 ±10%

✅ Acceptable Range:

👉 207 V – 253 V

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⚠️ What This Means

👉 Below 207 V → Undervoltage problem
👉 Above 253 V → Overvoltage problem

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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:

👉 Is this measured at true no-load?
👉 Is the voltage stable or fluctuating?
👉 Are all phases balanced (3Φ)?
👉 Is there a supply issue upstream?
👉 Could this affect equipment performance?

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⚠️ Real Risks

High Voltage:

🚨 Equipment damage
🚨 Overheating
🚨 Reduced lifespan

Low Voltage:

🚨 Motors draw more current
🚨 Poor performance
🚨 Equipment failure

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⚠️ Common Mistakes

• Measuring under load
• Not testing at point of control
• Not checking all phases
• Ignoring tolerance limits
• Assuming supply is correct

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💡 Final Thought

Test 9 is simple…

But it confirms something critical:

👉 The installation is being supplied correctly before use.

At TDMI Training, we emphasise:

👉 Never assume supply conditions
👉 Always verify at the source

Because the issue might not be your wiring…

👉 It might be what’s feeding it.

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✅ The Standard

• Measure at point of control
• Ensure no-load condition
• Test correct points (L–N / L1–L3 to N)
• Confirm within 207 V – 253 V
• Record results

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👉 Because correct voltage is the foundation of safe operation.