The electrical safety standards in the UK state that the resistance of the earth conductor of and class 1 piece of equipment must be lower than 1 Ohm (I think, it might be even lower).
Some soldering irons are powered off a transformer which is best for both ESD and safety alike. The transformer's voltage is always under 50VAC (normally 12V or 24V) so the soldering iron is considered to be a class 3 device which requires no earth connection, although some models connect the tip to earth via a 1M or 10M resistor to limit the current of ESD as well as protecting the transformer.