120VAC vs 24VDC

BryanG · Mar 6, 2020

It is a common misconception that 24V d.c is safe. If you are in a wet area 24V d.c. can most definitely kill you. Doesn't matter if it is 24V d.c. or 110V a.c. the only way to be safe is to not touch the live parts. At least the RCD/ELCB, and whatever acronym you use in the US, devices to protect people from accidental contact with a.c. voltage are cheap and easy to fit.

JesperMP · Mar 6, 2020

BryanG said:
It is a common misconception that 24V d.c is safe. If you are in a wet area 24V d.c. can most definitely kill you. Doesn't matter if it is 24V d.c. or 110V a.c. the only way to be safe is to not touch the live parts. At least the RCD/ELCB, and whatever acronym you use in the US, devices to protect people from accidental contact with a.c. voltage are cheap and easy to fit.

?
You have to elaborate.
In the EU, using an "extra low voltage" i.e. below 50V is an accepted method (amongst other things) to protect against electric shock, incl. in wet areas.

BryanG · Mar 6, 2020

Sorry for the delay in reply, busy morning.

BS EN 60204-1, don't have the most up to date, but I doubt this will have changed much.

Code:

6.4    Protection by use of PELV

6.4.1    General requirements

The use of PELV (Protective Extra-Low Voltage) is to protect persons against electric shock
from indirect contact and limited area direct contact (see 8.2.5).

PELV circuits shall satisfy all of the following conditions:
a) the nominal voltage shall not exceed:
-    25V a.c. r.m.s. or 60 V ripple-free d.c. when the equipment is normally used in dry
locations and when large area contact of live parts with the human body is not
expected: or
-    6V a.c. r.m.s. or 15V ripple-free d.c. in all other cases;

JesperMP · Mar 6, 2020

You are right that it is not enough with 24V to be safe in a wet area.
I should have mentioned that I mean SELV, not just ELV.
Basically SELV is "an electrical system in which the voltage cannot exceed ELV under normal conditions, and under single-fault conditions, including earth faults in other circuits", whereas PELV is "an electrical system in which the voltage cannot exceed ELV under normal conditions, and under single-fault conditions, except earth faults in other circuits"

In EN 60204-1 Chapter 6.1 it says:

The measures for protection given in 6.2, 6.3, and, for PELV, in 6.4, are a selection from IEC 60364-4-41. Where those measures are not practicable, for example due to physical or operational conditions, other measures from IEC 60364-4-41 may be used (e.g. SELV).

So SELV is safer than PELV, and I can use SELV if for some reason PELV is not enough.
IMO, one must forget about PELV because how can you exclude earth faults in other systems ?
Also, SELV rated power supplies are common and not expensive. I bet that most 24V PSUs that we use are actually SELV rated.

BryanG · Mar 6, 2020

I agree that SELV is safer than PELV but there are panel and machine design considerations that have to be taken in to account. SELV known as Safety Extra Low Voltage or Separated Extra Low Voltage, and it is the Separated bit that involves design. Just grabbed this of the well known source of always good information

Code:

Safety by extra low voltage SELV is used in situations where the  operation of
electrical equipment presents a serious hazard (swimming  pools, amusement
parks, etc.). This measure depends on supplying power  at extra-low
voltage from the secondary windings of isolating  transformers especially designed
according to national or to  international (IEC 60742) standard. The impulse
withstand level of  insulation between the primary and secondary windings is
very high,  and/or an earthed metal screen is sometimes incorporated between the
windings.
  
 The secondary voltage never exceeds 50 V rms.

 Three conditions of exploitation must be respected in order to provide
satisfactory fault protection:
  
No live conductor at SELV must be connected to earth
Exposed-conductive-parts of SELV supplied equipment must not be connected
to earth, to other exposed conductive parts, or to  extraneous-conductive-parts
All live parts of SELV circuits and of other circuits of higher voltage
must be separated by a distance at least equal to that between the primary
and secondary windings of a safety isolating transformer.

I am happy to argue whether to earth the output of a power supply, but this would involve not earthing any part of a machine. Or is my ever reliable source of information a bit confused.

JesperMP · Mar 6, 2020

It was my mistake to use 24 and (S)ELV as an example of why I disagree with your statement "Doesn't matter if it is 24V d.c. or 110V a.c".
It has derailed the discussion which was not my intention.

Aabeck · Mar 6, 2020

For extreme safety: specify all battery powered wireless sensors.

Then make sure you get the contract to provide all the batteries.

BryanG · Mar 6, 2020

Sorry Rson, we got a bit involved in our own discussion. I just wanted to explain that 24V d.c. isn't necessarily safe, as the saying goes 'it isn't the voltage that kills you but the current'. So if you have a low skin resistance naturally or it is lowered by where you are working, 24V d.c. can still kill you.

For any sensors outside the box I use 24Vd.c., unless it is somewhere wet. For inside the box I use what is most convenient and as in Europe we normally have a Neutral giving us 240V a.c. I use that for contactor coils. I did a job going out to the states where there was no Neutral and then used 24Vd.c for all the controls. I understand the argument for always using 24V d.c. on contactors but as there is 415V a.c. a terminal or two away what is the advantage.

Rson · Mar 6, 2020

Not a problem.

That is why in the first post I stated:

24VDC has a near-zero chance of being lethal.

Near zero is low, but still possible.

BryanG · Mar 6, 2020

Yeah, except I am one of those people with a low skin resistance. Thinking about electrical engineer probably wasn't the smartest career choice. They have torture screens in films using a couple of car batteries, I would be screwed if they had a PP9. Haven't tried it for years but I used to be able to touch the leads on a voltmeter and get a reading.

Aabeck · Mar 6, 2020

BryanG,

The only major shock I ever got was from a car battery. It was raining and the car wouldn't start. I opened the hood to check the terminals on the battery to see if they were tight. First the negative terminal was OK.

Then, with one hand resting on the radiator I grabbed the positive terminal and got one large 12 volt shock that hurt and made both arms numb for about 15 minutes. That was with the engine off and no battery charger connected.

As for the voltage reading on the skin, I can get from 100 to over 300 mV if I take a reading on the same finger about 1/2" (12mm) apart, less from closer or larger distances, one finger to another or another area of the body.

wes.s · Mar 6, 2020

I have a bunch of automated part washers from differing companies and they all use 120vac for the sensors inside. The DC and electrolysis comment was interesting, maybe that is why.

As far as low voltage, burned my hand once from a 9.6v strobe battery pack. I was doing some film developing at the time so my hand likely had something on it that make my skin surface conductive. I got a steam burn from the 9.6v when my hand bridged the contacts while I was changing out packs in the charging unit.

saultgeorge · Mar 6, 2020

"I'd say the best argument would be, how long do you want to be down? Ever try and troubleshoot a panel that has no power?"

Yes, and it's nearly impossible. I've worked for companies that have, for want of a better description, an "oppressive" safety program; a "guard over a guard" type situation. I have argued for hours that OSHA and the LOTO programs allow circuits to be energized for troubleshooting purposes. I suppose one needs PPE now, as mentioned above for more than 50 volts, but one can work it hot for troubleshooting. 24 VDC eliminates all of the above.

JaxGTO · Mar 6, 2020

I always liked 120vac but with almost all the plants enforcing PPE now I have moved to the 24vdc camp.

I worked on a machine recently that had all 24vdc using sourcing inputs and sinking outputs. That makes it real safe for a shorted wire. If you short in input wire all it does is turn on the input. If you short an output wire its just 0vdc to 0vdc (if you ground the 0vdc which they did).

rQx · Mar 6, 2020

I do troubleshooting on machines with 230vac control and 400v motors all the time with power on, so cant really relate to all the comments about that.

But still a lower voltage is safer no matter how you see it, that beeing said we use 230vac control on smaller machines regulary.

120VAC vs 24VDC

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