โก SANS 10142 Testing Principles Series
Test 10: Voltage With Load โ Verifying Voltage Drop Under Real Conditions
Introduction
Up to now, weโve established:
- Test 9: The correct voltage at no load (baseline supply)
Now we ask the next critical question:
๐ What happens to that voltage when the system is actually in use?
Because voltage is not staticโฆ
๐ It drops when current flows.
๐ง 1. Foundation (Understanding): What Are We Actually Testing?
This test measures:
๐ The voltage at the point of control under load conditions
And compares it to:
๐ The value measured in Test 9 (no load)
๐ What This Test Proves
๐ That voltage drop is within acceptable limits
๐ That conductors are correctly sized
๐ That the installation performs under real conditions
โ ๏ธ Why This Matters
If voltage drop is too high:
๐จ Equipment malfunctions
๐จ Motors draw more current
๐จ Overheating occurs
๐จ Efficiency drops
๐ ๏ธ 2. Application (Doing): How Must the Test Be Done?
โ ๏ธ IMPORTANT
๐ Test is done at the same point of control as Test 9
๐ But now with load applied to the installation
๐ง Correct Method
- Take your Test 9 reading (No Load)
- Apply load to the installation
- Measure voltage again at the same point
- Compare the two values
โก Measurement Points
Single Phase (1ฮฆ):
๐ L โ N
Three Phase (3ฮฆ):
๐ L1 โ N
๐ L2 โ N
๐ L3 โ N
๐ Voltage Drop Calculation
๐ Always compare:
Test 9 (No Load) vs Test 10 (With Load)
โก Example (Realistic)
- Test 9 (No Load): 238.8 V
๐ Maximum allowable drop:
- 5% of 238.8 V = 11.94 V
๐ฏ Minimum Acceptable Voltage Under Load:
๐ 238.8 V โ 11.94 V = 226.86 V
โ Final Interpretation
๐ Under load, voltage must NOT drop below:
๐ฅ 226.86 V
โ ๏ธ If It Drops Below This
โ Voltage drop exceeds 5%
โ Installation is not compliant
Possible causes:
- Undersized conductors
- Long cable runs
- Poor connections
- Overloaded circuits
โก 3. Mastery (Owning Responsibility): What Are the Limitations?
This is where professionals stand out.
๐ The Professional Must Ask:
๐ What load is applied during the test?
๐ Is it representative of real usage?
๐ Are all circuits contributing to voltage drop?
๐ Is the supply already weak (from Test 9)?
๐ Are long cable runs affecting results?
โ ๏ธ Real Risks
Excessive voltage drop leads to:
๐จ Poor equipment performance
๐จ Increased current draw
๐จ Heat build-up in conductors
๐จ Premature equipment failure
โ ๏ธ Common Mistakes
- Not referencing Test 9 value
- Measuring at different points
- Not applying sufficient load
- Ignoring 5% limit
- Assuming supply is the issue
๐ก Final Thought
Test 10 connects theory to reality.
Because this is where we see:
๐ How the installation behaves when it is actually used
At TDMI Training, we emphasise:
๐ Donโt just test voltageโฆ
๐ Test performance under load
โ The Standard
- Use Test 9 as baseline
- Measure at same point
- Apply realistic load
- Calculate voltage drop
- Confirm within 5% limit
๐ Because voltage that looks good with no loadโฆ
must still perform when it matters.
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