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I’m learning about electrostatic discharge and I’ve read that my soldering iron should have a ‘grounded tip’ which I think means its power plug should have an earth prong which it does.

I took a resistance reading i.e. I attached one DM probe to the soldering iron tip and the other DM probe to its earth prong. The reading is 9.4 mega ohms. I read that the resistance reading should be close to zero to indicate the iron is grounded. Since it far from zero (9.4), does that mean my iron is not grounded?

Thanks

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    \$\begingroup\$ It may be grounded through a 10M resistor, which is also common practice. \$\endgroup\$
    – Hearth
    May 31, 2021 at 1:43
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    \$\begingroup\$ The resistor is small enough to prevent damage from static buildup, and large enough to prevent damage if the iron tip should contact a live circuit, e.g., a hot mains wire. \$\endgroup\$
    – DrMoishe Pippik
    May 31, 2021 at 2:04
  • \$\begingroup\$ That resistor is there to prevent welding the iron to your circuit amidst a fireworks display. If you would inadvertently touch the grounded iron to a hot circuit bad things can happen. It also protects your project if you accidentally touch a electrified part. Grounding things in circuits can destroy them. A lot of power supplies and test equipment is connected to ground. \$\endgroup\$
    – Gil
    May 31, 2021 at 2:58
  • \$\begingroup\$ Thanks Hearth, DrMoishe Pippik, Gil, and Sam Gibson for your helpful and insightful replies. I'm going with the notion that there is a fixed resistor within the iron to protect its internal circuity. \$\endgroup\$
    – danarch
    May 31, 2021 at 23:17

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The reading is 9.4 mega ohms. I read that the resistance reading should be close to zero to indicate the iron is grounded.

The phrase sometimes used would be "hard-grounded" for a very low resistance to ground.

So what could cause your 9.4 MΩ measurement between the soldering iron tip and mains earth pin? As commented by Hearth, the most likely cause is a fixed resistor within the soldering iron, between the tip and the incoming mains earth connection. I've only seen 1 MΩ resistors used there, but 10 MΩ is another reasonable value.

I wrote about various soldering iron grounding options in another answer here:

Bonding in a soldering station

I won't repeat all of that (do, please, read that too) but I'll just point out that there is a significant disadvantage of a "hard-grounded" soldering iron tip, in an imperfect bench setup.

Let's say you have some components on a bench, without an ESD mat, and without any means to gently "bleed-off" any charge they have acquired from nearby insulators (plastic coffee cups etc) which shouldn't be there. (This is known as charging by electrostatic induction.) If you then start to solder those components, if your soldering iron tip has effectively zero resistance to earth, you can cause a damaging ESD event as your hard-grounded soldering iron tip touches a charged component. That accumulated static charge flows to earth quickly through the soldering iron, due to the lack of a high-value (e.g. 1 MΩ or 10 MΩ) resistance to reduce the instantaneous ESD current.

That is why, in many cases, a resistance like the 9.4 MΩ you measured between the soldering iron tip and ground (e.g. mains earth) can be preferable to a "hard-grounded" tip.

You may be able to solve the mystery in your case by unplugging your soldering iron, waiting for it to go cold, and opening it to see if you can find a physical resistor between the soldering iron tip and the mains incoming earth connection.

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