# Why is there a voltage between the mains ground and my radiator?

I have an anti-static mat that comes with a plug that only has a ground lead. Out of curiosity, I measured the voltages between the live, neutral, ground, and my radiator.

As expected, there is 240VAC between live and neutral, and in my case there is no AC voltage between neutral and ground. However, I measure 1VAC and 20mV DC between ground and my radiator.

I understand that there might be a difference between neutral and ground due to wire resistance and large return currents on the neutral wire. What I don't understand is where the potential difference between ground and my radiator comes from. It is my understanding that any ground current would trip the RCD.

As a follow-up question: What is the better location to connect my anti-static mat? I would prefer to not expose myself to random voltages, even though I understand it should not matter too much as long as I'm not grounding myself to two different potentials.

• What type of radiator, electric, gas or water? – HandyHowie Mar 12 at 9:08
• RCDs don't measure ground current. They measure the difference between live and neutral currents. That means that they don't care about stray currents creeping in from elsewhere. If you have metal supply pipes, those currents can even come in from neighbouring homes. – Simon B Mar 12 at 9:32
• Does your voltage meter offer the capability to switch to a "low resistance" mode? Typically they get an internal resistance of something like 100kOhm in this mode. You can check, if the voltage you see vanishes with this lower input resistance. – jusaca Mar 12 at 9:46
• Assuming electrical radiator - do you m assure any difference weather it’s in or off? – winny Mar 12 at 9:50
• @jusaca's comment deserves really to be a whole answer: false readings from modern high-impedance digital multimeters need to be kept in mind constantly. See dam-assets.fluke.com/s3fs-public/2105317_A_w.pdf, for example. But also: somewhere in your house the radiator piping, the water piping and the gas piping, if metal, really should be connected to electrical ground. Hardware stores will carry connectors and cable as mandated by local code. – CCTO Mar 12 at 15:23

I measure 1VAC and 20mV DC between ground and my radiator.

I would consider that negligible. Very likely there is a connection somewhere between your radiator and ground. It might be a long path though, for example:

Radiator - pipes to central heating unit - cold water pipes (does your heater also provide warm tap water?) - cold water grounding connection.

In my house at the place where the cold water enters the house, the water pipe is grounded together with the mains ground.

This means your radiator probably is grounded only through a very long path. Due to the length of this path there are all kinds of opportunities for it to pick up a small voltage.

I recommend grounding your ESD mat through the mains earth. Do make sure however that there is a 1 Mohm (yes, 1 Mega-ohm) resistor between the ESD mat and the actual ground. This is needed for your safety.

Suppose you have one hand on the ESD mat and your other hand touches a mains live voltage, the current must somehow be limited for your safety. That's the function of that 1 Mohm resistor. If you bought a "proper" ESD mat + ground connector, this 1 M resistor will be present already. But to be sure, I would measure it to confirm. For ESD that 1 Mohm resistor makes no difference at all, charges can still flow to ground through it.

• Note that all standard anti-ESD wrist straps should have a 1 Megohm resistor in their cable. You can plug this into the ground point on your ESD mat - even though this has its own 1 Mohm resistor in its path to ground. 2 Mohm will still work fine for grounding static charges. – Jamie Hanrahan Mar 12 at 12:07
• Thanks. My reasoning was that while the radiator may have a long ground path, the mains ground runs alongside the live wire, maybe making it more likely to pick up interference. I checked, and my mat measures 100KOhm – Pepijn Mar 12 at 12:08
• As implied, "Earth Ground" isn't the same everywhere -- this is why machine-shops ground machines at the machine, and drive a spike through the floor to the earth ground. Grounding it on the other end of the building could actually pick up voltage, and that would be dangerous for components plugged into the machine, and people operating it. (This is also why we use Fiber for LAN connections from building-to-building anymore: different potential energies, Fiber eliminates that.) – Der Kommissar Mar 12 at 14:37

One reason for potential differences might be that your measuring device closes a ground loop.

Basically there is a conductive path formed in your house that encloses an area where there is a changing magnetic flux. Due to the ever-present 50Hz (60Hz in some areas) field this conductive path becomes an antenna for this signal, which will give voltages and currents of the order you are measuring.

This is why you must only have one connection to ground.

The ground current does not trip breakers, as differential breakers measure the current difference between the live and neutral wire, the ground wire is not taken into account.

The best location for the grounded mat is right where you work ;-) Its purpose is to prevent the build-up of static charge, and that it will do at any location as long as plugged in to the ground.