Voltage at soldering iron tip, should I be worried?
Voltage at soldering iron tip, should I be worried?


Summary:


Don't be worried, unless the voltage is more than say 10 millivolts. In that case, just consider it to be unsuitable for use with electronics, put it to some other use or discard it, and get a "real" one.  
It is common enough for low quality soldering irons to have voltage on their tips due to leakage.  This is unacceptable and can and does do damage when soldering suitably delicate equipment.    
Reputable soldering iron makers take care that their products do not have this problem. 




Here is a useful document that discusses tip leakage and other soldering iron factors. Note that where they MAY APPEAR to have written "MV" the original was "mV" (snall m) for milliVolts and the font is rendered incorrectly by your system if you see a large M (as I do). They note - 


Tip-to-ground potential: Sometimes called ‘millivolt leakage’. The voltage existing between the soldering iron tip 
and the workstation common point ground. It should not exceed 2mV per DOD-STD-2000; not specified in J-STD-
001B.
Tip-to-ground resistance: The resistance existing between the tip of the soldering iron and the workstation
common point ground. It should not exceed 5 Ohms per J-STD-001B, appendix A, and 2 Ohms per DOD-STD-2000.
Insulation resistance: The resistance between the soldering iron tip and its heating element (applies to Hakko
soldering irons, series 900, 903, 907, 908). Properly termed ‘isolation resistance', it is a measurement of the isolation 
of the tip from the transformer secondary winding. 


The last point implies a transformer in the system but an equivalent rating applies to 'direct connection to mains' irons. 



ESD association soldering iron test methods



Hakko soldering iron tester one of these you will not be buying. BUT it gives you some good ideas.

leakage voltage testing

tip to ground testing

Hakko maintenance inspection

Note that whereas paper above quoted 2V as acceptable level !!! :-( - Hakko so Mil Std is 2 mV. More likely. Hakko say - 


How much leak voltage is normal?:  The lower the leak voltage, the better.
The MIL standard which has been abolished at present specifies that the leak voltage is designated to be lower than 2mV; therefore, all HAKKO station-type soldering irons have been delivered with the leak voltage set to 2mV or less in conformance to the MIL standard.
The MIL standard is a US military-based standard and the standard MIL-STD 2000A regarding soldering irons has been abolished.

Measure leak voltage after adjusting the temperature setting to the maximum.
When the leak voltage is measured at the highest temperature, the highest leak voltage will be recorded. If normal results are obtained even in unfavorable conditions such as at the highest temperature, it is judged that normal results will be obtained at other temperature settings. (This method is based on the MIL standard.)