92 Note on Induced Voltage on an Ungrounded Robot Arm

1. Safety Warning

To comply with the IEC 61140 electrical safety standard and equipment usage specifications, AUBO explicitly requires: the robot arm must be connected to a qualified protective earthing conductor (PE line) before it can be powered on and used. Starting the equipment without connecting the PE line is prohibited.

Note

Protective earthing conductor: also known as the ground wire, hereafter referred to as the PE line.

2. Problem Description

During many on-site commissioning sessions, the AUBO technical team found that when the robot arm is not connected to the PE line and is in the powered-on state, using a digital multimeter to measure the metal parts of the robot arm housing (such as the base flange, joint housing, and end flange) can measure an induced voltage of 15V ~ 105V. However, after connecting the PE line, it immediately drops to <1V, and the electric-shock sensation disappears at the same time.

3. Cause Analysis

1. Coupling Effect of the Y Capacitor

   The switching power supply inside the robot arm is designed with a Y capacitor between the high-voltage side (primary) and the low-voltage side (secondary) in order to suppress electromagnetic interference (EMI) and meet electromagnetic compatibility (EMC) requirements. This capacitor provides a coupling path for AC signals, causing the weak voltage of the live wire to be coupled to the output terminal or the housing, thereby forming an induced voltage.

   Note

   Y capacitor: a mandatory safety component that complies with the IEC 60384-14 standard, and is not a design defect of the equipment.

2. Voltage Accumulation in the Floating State

   When the equipment is not connected to the PE line, the housing is in a floating state, and the charge generated by the above coupling cannot be discharged in time, resulting in an AC induced voltage of 15V ~ 105V between the housing and ground. This voltage is a "phantom voltage" with a very small current (microampere to milliampere level), which will not directly cause serious personal injury, but will produce a tingling or numb electric-shock sensation in the hand.

   Note

   Floating state: the equipment housing is not electrically connected to the ground, and is in an electrically isolated state, so the charge accumulated inside the equipment cannot be discharged through the ground, forming a detectable voltage to ground on the housing.

3. Discharge Effect of the PE Line

   After a qualified PE line is connected, the housing and the ground form an equipotential, and the charge coupled to the housing is quickly discharged to the ground through the PE line, so the induced voltage immediately drops to < 1V, eliminating the safety hazard.

4. Solution

Core measure: connect a compliant PE line.

1. Wire requirements: the PE line cross-sectional area must be ≥ 2.5mm² (copper-core cable), with no damage to the insulation and firm crimping.
2. Grounding resistance requirements: after the grounding construction is complete, use a ground resistance tester to measure between the robot arm PE terminal and the ground. A grounding resistance of ≤ 4Ω is qualified (complying with the IEC 61140 standard), and the test record must be kept on file.

5. Essence and Risk Warning

The induced voltage phenomenon is an inherent characteristic of switching power supplies. All switching power supplies and drives that comply with IEC/GB standards contain Y capacitors, and it is not an equipment fault.

Warning

When equipment using a switching power supply is not connected to the PE line, the induced voltage generated presents the following risks:

• It may cause operators to experience a numb, tingling, or other electric-shock sensation. Although the current of the induced voltage itself is very small and will not directly cause serious electric shock injury, the sensation can easily cause the operator to panic and misoperate (such as bumping the robot arm or accidentally triggering the emergency stop), leading to secondary injury.
• It may cause potential damage to electrostatic-sensitive electronic components, cause faults in the robot control cabinet, and cause substantial damage to the power supply unit.
• It does not comply with electrical safety specifications and presents a safety hazard.