Have you ever encountered a situation where, before disconnecting a terminal, a voltage tester indicates that the circuit is safe, yet you receive an electric shock during or after the disconnection? Some may suggest that the voltage tester was inaccurate or that power was suddenly restored. While this possibility exists, in most cases, the issue lies with the line you’re disconnecting.
Let’s explain this with a basic circuit principle. As shown in Figure 1, when switch S is closed, and the load is operating normally, a voltage drop occurs across the load, and points A and N are at the same potential. The load is directly connected across L/N. In this case, the voltage tester, measuring between A and N, will show no voltage, indicating a safe condition.
However, if you proceed to disconnect the terminal at point A (or its equivalent position), this is equivalent to opening switch S, as shown in Figure 2. With the circuit now open, the load current becomes zero, and point A becomes at the same potential as L. At this point, the voltage tester will measure the rated voltage across point A and N, which is dangerous.
Yes, if you happen to disconnect point A while the system is still powered, you might encounter a shock, even though the voltage tester showed no voltage before disconnection. In real-world scenarios, when performing coordinated line disconnections, the entire control circuit is often not visible, and many workers simply test the voltage before starting the disconnection, believing it’s safe. However, theoretical analysis proves that testing voltage before disconnection alone is not safe.
Safe Procedures:
To ensure safety, at least the following steps should be followed:
Confirm that the system is de-energized before starting disconnection work.
Perform voltage testing both before and after disconnection. Do not touch cables, terminals, or conductive parts unless voltage testing has been carried out in both steps.
Note: This analysis applies to grounded power systems. For ungrounded systems, similar principles can be applied.