Why is the message "line defective" for socket strips with switch and control lamp, but not when you turn the plug through 180 degrees?
This is due to the lamp, as this represents a load between L and N. As a result, this is no longer a pure line.
Since the EL1 is constructed asymmetrically to detect a reverse polarity - for other leads one needs that, e.g. for IEC connection cables - the EL1 function in a plugging direction causes the lamp to burn properly. This then generates the error message.
In the other plug-in direction, the lamp lights up weaker and that does not bring the EL1 function to respond. That's why it works in one direction and not in the other. However, there are also lamps in these control switches, which bring the EL1 function in both directions to an error message.
Unfortunately, you can not do anything here. It also helps nothing to put the iso voltage to 250V down. This works only for the isomeasurement, in the continuity test of the EL1 always 500 V are used. The 250 V help but very well, if the part also has a surge protection.
If nothing helps, then it must be tested without the EL1 function, ie as a completely normal device. The VDE tests are thus taken into account, only a continuity test of L and N is not performed.