Sous Vide - Dangerous Voltages?

Hi Guys!

I’m trying to make a Sous Vide machine as described at:

I’m concerned about the safety of the project, mainly 120V/220V that is easy to touch. I’m in Asia where we use 220V.

When wall power is disconnected, I measured:
Heater Terminal 1 → Heater Terminal 2 = about 120 Ohms
Terminal 1 = Live, Terminal 2 = Neutral
Hotplate Terminal 1 → Hotplate Surface = Seems to be disconnected (Over several Megaohm Resistance - Over Limit)
Hotplate Terminal 2 → Hotplate Surface = Seems to be disconnected (Over several Megaohm Resistance - Over Limit)
Hotplate Surface → Ground = Connected (Continuity Test good)

When I measure AC Voltage from:
Power socket Ground → Multimeter Black, Multimeter Red → Me = 90Volts AC ← Is this strange? I have no idea

Now, getting the system going:
Fill pot with water and insert into rice cooker
Plug rice cooker into wall
Water in pot → Multimeter Red, Ground → Multimeter Black = 120V AC ← Whoa… am I going to get killed if I touch the water?
My hand → Multimeter Red, Ground → Multimeter Black = 200V AC ← Wait just a minute here… what’s goin’ on?

Now granted 120V AC is not 220V AC. But it’s certainly enough to kill. Did my pot somehow get electrified even though the hotplate surface is ground?

Anyways… since it’s getting late, I grabbed a piece of wire and stuck it in the water and touched the end of the wire with my pinky… I didn’t die or feel any shock.

I still can’t explain any of these AC voltage readings… but I’d really like to hear from anyone who can help me understand:

  1. How to tell if I really did have AC current running into the water
  2. What these voltage readings mean (For example, do I have a bad ground? Is it normal to see 120V popping up because… it’s one of the AC sine waves in one of the wires…

Thanks in advance guys!

Jeff

All voltages are relative. There can be a difference in voltage between you and ground of hundreds of thousands of volts (ever get a static electricity shock from a car or whatever?)

Also, there should be no relation between electrical socket ground and the socket live/neutral. If any reading between ground and live/neutral makes sense (ie. 0V or 220V) then the socket isn't wired correctly (or something further up the line is wrong).

The only important thing is that anything you can touch with your hand is connected to ground. If that's true then you're safe. PS: Make sure the ground pin of your electric socket is properly connected...

Thanks a bunch for the info!

Magnetic induction causes wires to change in voltage. It is common to put a multimeter on an extension cord that is not plugged in and measure high voltage because invisible magnetic flux is changing around the wire.

"1) How to tell if I really did have AC current running into the water"

The current is measured with an ammeter not a volt meter. Use the multimeter to measure current and that will squelch the voltage. The current will be insignificant.

"2) What do these voltage readings mean ?"

The voltage means the wire is floating. The water is electrically floating. Your hand is floating. Invisible magnetic flux linkages and electrical fields are causing voltages to be measurable on your hand, your water kettle.

"(For example, do I have a bad ground? "

No bad ground is needed to find a space charge or dark forces. A good ground will make those ghost voltages flee to the nether regions of immeasurable distance and invaluable safety.

"Is it normal to see 120V popping up?"

Yes, it is perfectly natural. Enjoy!

fungus:
All voltages are relative. There can be a difference in voltage between you and ground of hundreds of thousands of volts (ever get a static electricity shock from a car or whatever?)

Also, there should be no relation between electrical socket ground and the socket live/neutral. If any reading between ground and live/neutral makes sense (ie. 0V or 220V) then the socket isn't wired correctly (or something further up the line is wrong).

Then I suggest you connect live to ground and see the fuse blow! Live is always at mains voltage above ground (0V). Neutral can be anywhere in between, but usually just a few volts above ground. There's sufficient difference between neutral and ground to trip my RCD, if they're connected together.

PS: Make sure the ground pin of your electric socket is properly connected...

Good advice. If the ground is properly connected, he should be able to read the full mains voltage between ground and live.

fungus:
Also, there should be no relation between electrical socket ground and the socket live/neutral. If any reading between ground and live/neutral makes sense (ie. 0V or 220V) then the socket isn't wired correctly (or something further up the line is wrong).

Can you clarify what you mean by this?

I would expect to get a very stable reading of 0V when reading between Ground and Neutral. In the USA, at least, Neutral and Ground are NOT connected in the socket, but they are wired to the same Ground/Neutral bar back inside the breaker box.

The difference is that Neutral will always have exactly the same current flow as Hot, except opposite direction (same phase), and Ground should NEVER have current unless there is something wrong.

Neutral is to complete the circuit, Ground is for safety.

polymorph:
I would expect to get a very stable reading of 0V when reading between Ground and Neutral. In the USA, at least, Neutral and Ground are NOT connected in the socket, but they are wired to the same Ground/Neutral bar back inside the breaker box.

They shouldn't be.

Your ground might not be the same as your neighbor's ground. It almost certainly isn't the same as the power company's ground. If you connect your neutral to ground then 'neutral' might not be in the center of the sine wave being output on the live wire by the power company.

"Ground" is for safety only, it's not a voltage reference.

fungus:

polymorph:
I would expect to get a very stable reading of 0V when reading between Ground and Neutral. In the USA, at least, Neutral and Ground are NOT connected in the socket, but they are wired to the same Ground/Neutral bar back inside the breaker box.

They shouldn't be.

....

here, in France, they are ! Not in your house, but in the company transformers.
with an exception for hospitals where there are special needs.

AmbiLobe:
Magnetic induction causes wires to change in voltage. It is common to put a multimeter on an extension cord that is not plugged in and measure high voltage because invisible magnetic flux is changing around the wire.

No, its the electric near-field, not the magnetic that is responsible for mains voltage being picked up by nearby
objects - if you do the maths you'll see that the live potential is varying at 75kV/s or so, whereas the flux changes are
far less (induced voltage equal to rate of change of flux - 120V would need a flux amplitude of 0.4 weber, and since
the fields produced by typical mains currents are around a milli tesla as close as 1mm, this isn't going to be enough
magnetic coupling unless the wires run that close for hundreds of metres.

Capacitive coupling on the other hand is perfectly plausible. In fact when you have isolated conducting objects
near to one another you have a capacitive divider network. Adding conducting paths to ground stops the coupling
by carrying the capacitive currents to ground (they are very small typically, microamp range, at mains frequencies).

Typical multimeters are very high impedance when on voltage ranges, so you get to see the capacitive potentials
(although they will affect them too).

alnath:
here, in France, they are ! Not in your house, but in the company transformers.
with an exception for hospitals where there are special needs.

It's OK to connect them right after they go through a transformer, but not in multiple places which are spaced apart.

fungus:

polymorph:
I would expect to get a very stable reading of 0V when reading between Ground and Neutral. In the USA, at least, Neutral and Ground are NOT connected in the socket, but they are wired to the same Ground/Neutral bar back inside the breaker box.

They shouldn't be.

Your ground might not be the same as your neighbor's ground. It almost certainly isn't the same as the power company's ground. If you connect your neutral to ground then 'neutral' might not be in the center of the sine wave being output on the live wire by the power company.

In the USA, Neutral and Ground absolutely are connected inside the breaker panel. But your local Neutral -and- Ground are NOT the power company's, either. There is a transformer that steps down to 240V centertap (again, USA, 120V to most sockets, 240V to a few things) and the centertap is the neutral -and- you will also have a local ground rod driven into the ground.

I can't speak for anywhere else.

My experience: I took electrical drafting in high school (longer ago that I care to say), I've done some wiring in my own homes, and a few years ago I shared a classroom with a bunch of Electrical Engineering Technician students while I was taking Electronic Engineering Technician courses. In my limited experience, I've never seen a modern panel (ie, with breakers) without a bussbar that has both Neutral and Ground connected together.

But I'd not expect you to take my word without references:

"A neutral wire is the return conductor of a circuit; in building wiring systems the neutral wire is connected to earth ground at least at one point. North American standards state that the neutral is neither switched nor fused except in very narrowly defined circumstances. The neutral is connected to the center tap of the power company transformer of a split-phase system, or the center of the wye connection of a polyphase power system. American electrical codes require that the neutral be connected to earth at the “service panel” only and at no other point within the building wiring system."

I cannot speak for any other country, nor did I try.

"Ground" is for safety only, it's not a voltage reference.

You cut out the part where I said that ground is for safety. I've never advocated using house wiring Ground as a voltage reference.

alnath:

fungus:

polymorph:
I would expect to get a very stable reading of 0V when reading between Ground and Neutral. In the USA, at least, Neutral and Ground are NOT connected in the socket, but they are wired to the same Ground/Neutral bar back inside the breaker box.

They shouldn't be.

....

here, in France, they are ! Not in your house, but in the company transformers.

Likewise in the UK. There is a connection between neutral and ground, not inside your house, but just outside it, to connect the power companies 'return' to your neutral. Three phase power supply cables (in the road) have only three conductors.

polymorph:
I would expect to get a very stable reading of 0V when reading between Ground and Neutral.

Not always the case. It depends on the relative resistance paths and the current the neutral wire is carrying. There is usually a small PD between them, enough for my RCD to trip if they come into contact.

In the USA, at least, Neutral and Ground are NOT connected in the socket, but they are wired to the same Ground/Neutral bar back inside the breaker box.

In the UK, they are kept seperate until they are connected in the Power Supply company’s cable (before the meter and main fuse). That cable’s neutral is grounded under the road. The live is connected to one of the three phases in the three conductor supply cable under the road.

The difference is that Neutral will always have exactly the same current flow as Hot, except opposite direction (same phase), and Ground should NEVER have current unless there is something wrong.
Neutral is to complete the circuit, Ground is for safety.

Agreed.

It sounds like USA and UK ground and neutral wiring isn't that different. Your's is just grounded a bit further from the house.

polymorph:
American electrical codes require that the neutral be connected to earth at the “service panel” only and at no other point within the building wiring system."

So ... why bother having three wires? You could get rid of neutral and just use earth instead.

Henry_Best:

polymorph:
I would expect to get a very stable reading of 0V when reading between Ground and Neutral.

Not always the case. It depends on the relative resistance paths and the current the neutral wire is carrying. There is usually a small PD between them, enough for my RCD to trip if they come into contact.

Which is as it should be (in my book, but apparently not in anybody else's). A short between neutral and ground in an appliance is a wiring fault.

Maybe enough idiots were connecting neutral to ground inside their fuse boxes that the power companies had to adapt and it became a de-facto standard.

fungus:

polymorph:
American electrical codes require that the neutral be connected to earth at the “service panel” only and at no other point within the building wiring system."

So ... why bother having three wires? You could get rid of neutral and just use earth instead.

Henry_Best:

polymorph:
I would expect to get a very stable reading of 0V when reading between Ground and Neutral.

Not always the case. It depends on the relative resistance paths and the current the neutral wire is carrying. There is usually a small PD between them, enough for my RCD to trip if they come into contact.

Which is as it should be (in my book, but apparently not in anybody else's). A short between neutral and ground in an appliance is a wiring fault.

Maybe enough idiots were connecting neutral to ground inside their fuse boxes that the power companies had to adapt and it became a de-facto standard.

have look at this http://www.cressall.com/neutralearthing/downloads/Types%20of%20neutral%20earthingIss2.pdf
Best protection with Solidly Earthed System

Isolated system is AFAIK, still used in hospitals because they need to continue operating even in a presence of a single earth fault .

alnath:
have look at this http://www.cressall.com/neutralearthing/downloads/Types%20of%20neutral%20earthingIss2.pdf
Best protection with Solidly Earthed System

Isolated system is AFAIK, still used in hospitals because they need to continue operating even in a presence of a single earth fault .

So why do we bother with three wires...? Two is safer!

fungus:

alnath:
have look at this http://www.cressall.com/neutralearthing/downloads/Types%20of%20neutral%20earthingIss2.pdf
Best protection with Solidly Earthed System

Isolated system is AFAIK, still used in hospitals because they need to continue operating even in a presence of a single earth fault .

So why do we bother with three wires…? Two is safer!

no, the neutral is connected to the ground in the transformer, not in the house. In the house, the differential protection detects when there is a fault (current in the Phase wire != currrent in the neutral wire => a part of it goes to the earth ), it wouldn’t be possible with only 2 wires .

alnath:

fungus:
So why do we bother with three wires...? Two is safer!

no, the neutral is connected to the ground in the transformer, not in the house. In the house, the differential protection detects when there is a fault (current in the Phase wire != currrent in the neutral wire => a part of it goes to the earth ), it wouldn't be possible with only 2 wires .

Like I said above "It's OK to connect them right after they go through a transformer" but everybody here is connecting neutral to ground in the fuse boxes of their house.

In double-insulated devices, there are only two wires. However, anything with a metal case (like a computer) is much safer with that case grounded.

fungus, have you bothered to look up the electrical code for Spain?

Do you understand why it is called a “safety ground”?