If I use 1 Mega Ohms series resistance, then is there any chance of electric shock from 220 Volts?

Thread Starter

pgmetgud

Joined Oct 9, 2018
3
If I directly touch a live 220 volts wire (standard mains supply in India), then I will get electric shock. But, let's suppose, if I insert one mega ohms resistor in series between my hand and the 220 volts wire, then is there any chance of getting an electric shock. Theoretically, I guess the current cannot be more than about 0.22 milliamperes. But, I am worried about resistor malfunction due to overheating or some other problem like that which I may have overlooked. I plan to use this method in a product where a "sensor-wire" is inserted in a water tank and even if by any chance the "sensor-wire" comes in contact with a 220 volts wire, the "one mega ohms resistor" will act like a safety device so that no person who touches the water will ever get any shock under any circumstances. I understand that a better way would be to totally isolate (may be by opto-coupling? ) the "sensor-wire" from the high-voltage-circuitry, but as that might increase the cost of my products, I am thinking is it enough to just use a one mega resistor as a safety device. Any Suggestions?
 

Yaakov

Joined Jan 27, 2019
1,604
Galvanic isolation is the only safe way to do this. You cannot depend on any system intended to isolate from high voltage that has a possible low impedance path to the mains.

Cost or not, don't try to cut corners on safety.
 

ericgibbs

Joined Jan 29, 2010
8,886
hi pq,
It is a bad idea, do not use that method.
Give us some more background information about your project, there maybe low cost safe alternatives.
E
 

crutschow

Joined Mar 14, 2008
23,542
I doubt whether a safety organization, such as UL, would approve such a scheme.
Such an approval is needed for selling any electrical device to the public.
 

MaxHeadRoom

Joined Jul 18, 2013
19,198
You will get a very high conduction with 220v in water, especially if not pure, you can use a much lower voltage to do this, and use isolation.
Max.
.
 

Yaakov

Joined Jan 27, 2019
1,604
You will get a very high conduction with 220v in water, especially if not pure, you can use a much lower voltage to do this, and use isolation.
Max.
.
Max, I think his idea is, if the electrode in the water is connected to the circuit via a large resistor, any accidental introduction of the mains onto the electrode via his product would not result in a dangerous electrode voltage.

I don't think he intends to intentionally supply 220V to it.
 

wayneh

Joined Sep 9, 2010
16,114
If I directly touch a live 220 volts wire (standard mains supply in India), then I will get electric shock. But, let's suppose, if I insert one mega ohms resistor in series between my hand and the 220 volts wire, then is there any chance of getting an electric shock. Theoretically, I guess the current cannot be more than about 0.22 milliamperes. But, I am worried about resistor malfunction due to overheating or some other problem like that which I may have overlooked.
On paper, and under controlled and safe conditions, you are correct that the 1MΩ resistor would protect you by limiting the current to 220µA. I think even that small current could be a problem if it were to pass directly across your heart, or perhaps to a person wearing a pacemaker. But in general it's not a big risk. The resistor would not be at risk from over-current, and resistors don't fail to a short, they tend to fail open and even that is rare.

In the real world, I would never do what you are proposing. Even though, in theory, it could be safe, Mother Nature and Murphy's Law are relentless in showing us new ways for things to fail. Being protected by "theory" isn't good enough. It's better to make the effort to add safety layers.
 

BobaMosfet

Joined Jul 1, 2009
839
Questions- is this AC or DC, and how much voltage and current does the sensor wire require? Exactly what is it sensing, water level? This will give us enough information to choose what is necessary to control current, and then you should use an opto-isolator to ensure true isolation away from mains.
 

Yaakov

Joined Jan 27, 2019
1,604
As the OP mentions directly touching 220v mains supply , then it's probably a safe assumption it is AC.
Max.
My current (or should I say voltage) understanding is this device will be mains powered and include a electrode being used as a sensor in water. I don't understand the TS's resistance to proper safety measures but this has the potential to be very dangerous so to insulate his company from liability he should properly isolate it.

Sorry, I couldn't resist. I mean it though.
 

Thread Starter

pgmetgud

Joined Oct 9, 2018
3
Galvanic isolation is the only safe way to do this. You cannot depend on any system intended to isolate from high voltage that has a possible low impedance path to the mains.

Cost or not, don't try to cut corners on safety.
My current (or should I say voltage) understanding is this device will be mains powered and include a electrode being used as a sensor in water. I don't understand the TS's resistance to proper safety measures but this has the potential to be very dangerous so to insulate his company from liability he should properly isolate it.

Sorry, I couldn't resist. I mean it though.
Thanks for your suggestion. I am a novice in this field. I will make a new design of the circuit so as to totally isolate the sensor wires from the product. And Yes, your guess is right...... this device design is mains powered. Actually, I had this new design already in mind, but I was hesitating because of extra cost. In this new design, perhaps I will use a small solar cell (of around 3 volts or even 6 volts) and a small rechargeable battery to activate the sensors. So, in such a design, user will not get a shock because it is only 6 volts DC. In the cities where this design is used, there is sunshine on almost all the days. During night time, the battery can supply power. I guess that would be ok. I am even contemplating on other designs in which the sensor wires are driven only by a low voltage power source which can never cause any electric shock in any situation. But, now I will make sure that there is galvanic isolation between the sensors and the device. I don't want some customer of mine to get a nasty fatal electric shock.
 

Tonyr1084

Joined Sep 24, 2015
3,676
So far we know 220 VAC, a container of water, some form of sensing but we don't know the purpose of the sensor. Are you activating a pump? Are you shutting off a pump? Operating a valve? What exactly is it you want to accomplish? Only when we know what you're actually doing can we give you sound advice. For us to make assumptions, we're making them with someone else's life at risk. There's a right way to do it. To start, knowing exactly what the task is. Then we can advise you on a safe and as cheap as possible solution.

From what little I can guess, you want to know when the tank is either full or near empty. Just exactly how the 220 VAC comes into play - - - maybe you're running a pump to keep a tank full. But why would your customers be coming in contact with the tank? The water in it? Till we know - we honestly don't know what you're doing.
 

Wolframore

Joined Jan 21, 2019
1,321
Rule of thumb was 100 mA causes ventricular fibrillation (they called it the funky chicken in school) so theoretically you should be fine. What’s not fine is to rely on a resistor around live mains. What happens if it shorts out?
 

Thread Starter

pgmetgud

Joined Oct 9, 2018
3
Then it would be interesting to know how/why the sensor would come in contact with 220V!
Max.
Actually the sensor is in touch with a 12 volt power supply via a relay. The relay gets switched ON only for a fraction of a second once every 10 minutes. That means, once every 10 minutes, the sensor is in contact with 12 volts. Now, this 12 volts is the output of a voltage regulator IC and input to that IC is from a small transformer which converts 220 volts mains supply to 12 volts and feeds it to the voltage regulator IC. I was afraid that, if by any chance the transformer coil gets burnt and if primary coil comes into direct contact with secondary coil, then that basically means the sensor is being touched with 220 volts directly. although that is difficult to imagine (because the above-mentioned relay-driven by that 12 volts itself- will automatically get switched OFF in case the transformer burns. thus it is theoretically impossible for sensor to ever get connected to 220 volts mains.). Although, just as you mention, it is hard to imagine why sensor would ever even touch the mains supply, I am a bit worried just in case if I missed something. One design I am thinking about is to use an 'isolation transformer" or may be i will use a fuse so that it will never allow any voltage more than 12 volts. Anyways, thanks for your valuable suggestion.
 
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