SANS 10142 Testing Principles Series (10)

⚡ SANS 10142 Testing Principles Series

Test 10: Voltage With Load — Verifying Voltage Drop Under Real Conditions

Introduction

Up to now, we’ve established:

• Test 9: The correct voltage at no load (baseline supply)

Now we ask the next critical question:

👉 What happens to that voltage when the system is actually in use?

Because voltage is not static…

👉 It drops when current flows.

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

This test measures:

👉 The voltage at the point of control under load conditions

And compares it to:

👉 The value measured in Test 9 (no load)

🔑 What This Test Proves

👉 That voltage drop is within acceptable limits
👉 That conductors are correctly sized
👉 That the installation performs under real conditions

⚠️ Why This Matters

If voltage drop is too high:

🚨 Equipment malfunctions
🚨 Motors draw more current
🚨 Overheating occurs
🚨 Efficiency drops

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

⚠️ IMPORTANT

👉 Test is done at the same point of control as Test 9
👉 But now with load applied to the installation

🔧 Correct Method

1. Take your Test 9 reading (No Load)
2. Apply load to the installation
3. Measure voltage again at the same point
4. Compare the two values

⚡ Measurement Points

Single Phase (1Φ):

👉 L – N

Three Phase (3Φ):

👉 L1 – N
👉 L2 – N
👉 L3 – N

📊 Voltage Drop Calculation

👉 Always compare:

Test 9 (No Load) vs Test 10 (With Load)

⚡ Example (Realistic)

• Test 9 (No Load): 238.8 V

👉 Maximum allowable drop:

• 5% of 238.8 V = 11.94 V

🎯 Minimum Acceptable Voltage Under Load:

👉 238.8 V – 11.94 V = 226.86 V

✅ Final Interpretation

👉 Under load, voltage must NOT drop below:

🔥 226.86 V

⚠️ If It Drops Below This

❌ Voltage drop exceeds 5%
❌ Installation is not compliant

Possible causes:

• Undersized conductors
• Long cable runs
• Poor connections
• Overloaded circuits

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

This is where professionals stand out.

🔍 The Professional Must Ask:

👉 What load is applied during the test?
👉 Is it representative of real usage?
👉 Are all circuits contributing to voltage drop?
👉 Is the supply already weak (from Test 9)?
👉 Are long cable runs affecting results?

⚠️ Real Risks

Excessive voltage drop leads to:

🚨 Poor equipment performance
🚨 Increased current draw
🚨 Heat build-up in conductors
🚨 Premature equipment failure

⚠️ Common Mistakes

• Not referencing Test 9 value
• Measuring at different points
• Not applying sufficient load
• Ignoring 5% limit
• Assuming supply is the issue

💡 Final Thought

Test 10 connects theory to reality.

Because this is where we see:

👉 How the installation behaves when it is actually used

At TDMI Training, we emphasise:

👉 Don’t just test voltage…
👉 Test performance under load

✅ The Standard

• Use Test 9 as baseline
• Measure at same point
• Apply realistic load
• Calculate voltage drop
• Confirm within 5% limit

👉 Because voltage that looks good with no load…
must still perform when it matters.