Hioki PD3259-90 Instruction Manual page 32

Attaching the Voltage Sensors to the Circuit
Improper attachment
If the clips are not attached properly, accurate measurement will be
impossible due to the effects of nearby wires.
The sensor is attached to the
insulated wire by the tip of the clip.
The insulated wire has been
inserted too far into the clip.
26
The sensor is attached to the
insulated wire at an angle.
The sensor has been clipped
simultaneously to two insulated wires
that are carrying different voltages.