Electrocution safety for 24V DC vs 120V AC

[GeorgeMfg]

Hi,

I am building a hot tub and have a question about electrocution risks. I have the option to use a 120v AC pump and a 24v DC pump, where wattage/power would be the same on both pumps. The spa tub is made from metal (don't ask why) and the tub itself is grounded. These pumps have all the electronics completely encapsulated, and the impeller is in a separate housing, but there is always a risk of something going wrong.

I've read that most spas actually use 220v water pumps. My preference would be to use the 120v AC pump but I have read that the 24v DC is safer. Can anyone explain why?

Are there any other considerations I should be making?

Thank you for your replies.

 

[wcaseyharman]

When the human body is wet the surface resistance decreases, numbers I’ve seem are upwards of 10k ohms when dry to around 1000 ohms when wet. It only takes 100 mA-ish to cause the heart to stop. 120/1000 is 120mA, which is above the range of a fatal shock. Lots of variables to be sure, but the potential (no pun intended) is there.

24V won’t cause a fatal shock under similar circumstances. Probably a good tingle but not too bad.

 

[dik]

Just a bit of a caution...

"Yes, but it’s practically impossible.

What kills is current, not voltage. Under all normal circumstances the insulation of your skin is too high for a 12 volt car battery to deliver enough current to any part of the body to deliver a fatal level of current.

But if you were to break the skin, and apply 12 volts across the heart, the conductivity within the body is very low, because blood for example is highly conductive, so then it’s possible to exceed the 100 mA that it normally takes to stop the heart. I understand that heart surgeons even use insulated instruments when working around the heart"

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[TugboatEng]

What kills is current, not voltage

This is a bullshit wives' tale that needs to die. Current can't flow without voltage. Resistance and capacitance in conjunction with voltage determine the amount of current that may or may not be fatal. Power is also a factor for determining the availability of a shocking current.

 

[che12345]

"...I've read that most spas actually use 220v water pumps. My preference would be to use the 120v AC pump but I have read that the 24v DC is safer. Can anyone explain why?..."
1. 24 Vdc is safer than 120Vac or 220Vac. 24 Vdc is unlikely to result to any fatal shock to any normal person, even with some minor skin cuts.
2. The only setback I can think of is the power supply authority does NOT offer 24Vdc . The step-down 120-220Vac/24Vac transformer windings in the rectification unit should be of "double wound" i.e. primary and secondary windings are separately wound on different limbs.
3. In addition, the 24Vdc can be designed with +12-0--12Vdc. With 0V earthed. Under any fault conditions to earth, the voltage is 12Vdc. Safety is further enhanced.
4 Attention : Use double-pole DC breaker on two lines. The 0V, earthed line need no switching.
Che Kuan Yau (Singapore)

 

[LittleInch]

For the pump which is normally separate from the hot tub and given its metal and earthed, I think your risk levels for any voltage are extremely low as the potential for anyone to actually come into contact with power leads is not there.

Lights inserted into the hot tub or anything where if it was broken could lead to wires plus water plus humans needs to be low voltage DC.

Really don't think this applies to a pump, especially if its double encapsulated.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[waross]

120 VAC versus 240 VAC?
Both systems use 120 Volts to ground in North America.
Ground Fault Circuit Interrupters are ubiquitous for 120 VAC circuits and are easy to install.
120 VAC GFCIs are available for panel mounting and for field mounting.
240 Volt GFCIs are less available and are less common for field mounting. (Don't show me catalogue listings, show me what's on the shelf locally.)
Shock hazard and broken skin:
I once designed and installed the wiring for an operating theatre. Most persons are not aware that in Canada there is a code (available at extra cost) governing wiring installations in operating theatres.
Your information is fairly good, dik.
At that time, dik, rather than insulated instruments, the conductivity of the floor surface was specified within imits as well as methods of verifying the floor resistance.
The floor resistance had to be high enough to limit the possible current returning to ground through the floor, but low enough to avoid static charges.
As for the supply transformer, the capacity was limited and while on the same limb, the primary and secondary windings were separated by a grounded metal shield.
And, by the way, DC is somewhat insidious.
On AC, you either feel a shock or not.
With prolonged contact with DC, the very low current caused by sustained contact with a low voltage DC will build up breakdown paths and the current will slowly increase with time.
I have received painful shocks from sustained contact with 6 Volts DC. (Long story, but it has happened more than once.)
You feel nothing for a long time and then suddenly the pain threshold is crossed and you get a jolt. Not dangerous on unbroken skin, but I wouldn't want that current to cross my heart.
For the hot tub, use 120 VAC and a GFCI.
I suspect that the 24 VDC option may have been for solar installations back when 24 VDC was common, and appliances were available for 24 VDC use on solar.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[LionelHutz]

Pput the system on GFCI protection.

Lots of spas use 120V or 240V and they are safe. I don't see the point of using 24VDC.

This is a bullshit wives' tale that needs to die. Current can't flow without voltage. Resistance and capacitance in conjunction with voltage determine the amount of current that may or may not be fatal. Power is also a factor for determining the availability of a shocking current.

Impressive. Rant about how it's not current then write 2 sentences which describe how you get to the right current to kill.

It's not bullshit that it is the current level reaching the heart that kills. There's a reason GFCIs trip when current reaches 4-6mA.

 

[crshears]

"The volts may give you the jolts, but it's the amps that give you the cramps . . . "

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Compositepro]

"What kills is current, not voltage"

There is a very good reason for this statement. Static electricity shocks are usually very high voltage at 5000 Volts and higher, but they are safe because the available current is very low. Electric fences and car ignition systems are the same. When electric shock is the goal, for deterence or muscle stimulation, high voltage must be used, and is perfectly safe when current is limited.

 

[davidbeach]

And when properly set up to do so, it is possible to work 500kV live. Lots and lots of voltage but no place for the current to flow. If it was just voltage that killed then birds couldn't sit on feeders.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982