12VDC VS 24VDC for PLC Controls

[Leslie Martin]

I am quoting a water park Install which has PLC controlled Items (i.e. Water Cannons) and I am having trouble with a Local Health Inspector who wants all of my control lines for the Pushbuttons and Solenoid valves to be 12VDC instead of 24VDC because of the shock hazard.
Is there any real difference between the 2 Voltages except for 12V.
If I run 24VDC the current required to close the solenoids will be lower then if I go with 12VDC and I think that would be more important.

Is there a Code Requirement which limits the voltage amplitutde in these situations? If there isn't how can I convince the Inspector 24V is better.

 

[petersonra]

There is no NEC requirement either way. It may be that it is just a preference. It seems to me that the person you should be asking is the engineer that is going to do the design work. While it is quite possible to get 12VDC stuff, it is not as easy as with the far more common 24VDC stuff.

 

[iwire]

Not real familiar with PLCs but would the 24 VDC source be grounded?

If it is not a grounded source you are not likely to get a shock from it. That is assuming you would get a shock from 24 VDC.

 

[cpal]

oIf you are using a plc why would the inputs drive load coils etc.????


Charlie

 

[iwire]

oIf you are using a plc why would the inputs drive load coils etc.????


Charlie

I was wondering the same thing....

Perhaps the inspector is requesting the outputs operate at 12 VDC as well?

 

[petersonra]

I see a lot of PLC systems now specifed as 24 VDC for operator safety. The systems I am designing now are 24 VDC, if we need to operator a contactor, then an interpoise relay is used.
However I have not seen (not saying they are not available) a PLC in 12 VDC
From a safety point there is no difference between a 12 vdc and 24 vdc system, as both will be a NEC Art 725 Class 2 circuit. A class 2 circuit limits the voltage and power for fire and shock protection.

Most 24VDC PLC I/O will work with a 12VDC supply. The usual spec is 10-50VDC.

 

[petersonra]

oIf you are using a plc why would the inputs drive load coils etc.????


Charlie

I did not see anything in the OP that said anything about inputs driving coils. ????

 

[petersonra]

Not real familiar with PLCs but would the 24 VDC source be grounded?

If it is not a grounded source you are not likely to get a shock from it. That is assuming you would get a shock from 24 VDC.

There are two schools of thought.

The first is to ground it. That way if you have a ground fault it will be detected and you can use a convenient chunk of metal as the return for voltage check during debugging operations.

The other is not to ground it so a ground fault does not cause loss of control.

It is a design decision usually based on convention in that particular location as much as anything.

One issue with 12VDC is that it is quite possible on long runs that an input will not function properly due to the capacitance on the line. it takes hundreds or thousands of feet to see this but it can and does happen. The only real answer is higher voltages.

 

[dereckbc]

One issue with 12VDC is that it is quite possible on long runs that an input will not function properly due to the capacitance on the line. it takes hundreds or thousands of feet to see this but it can and does happen. The only real answer is higher voltages.

Not sure what you are getting at here.

As one who works a lot with 12, 24, 48 VDC systems, and digital communications, the real problem with any low votage system is voltage drop. Especially on a 12 VDC system. Even modest distances like 50-feet can be problematic if any appreciable load current is involved causing one to way over-size circuit conductors to compensate for VD.

However I do not see problem for the pushbutton to operate some pump or PLC trigger as no current would be involved. All you need is a signal, not a load source to drive a realy or solenoid.

As to the ground reference is a none issue from NEC point of view, it is just not required at this voltage. Both grounded and floating have pros and cons. Grounded is less expensive and less complex, floating is safer and more reliable.

 

[iwire]

It is a design decision usually based on convention in that particular location as much as anything.

Thanks for the info Bob, I was not sure if ungrounded was an option. I was unsure if the PLCs had to utilize grounded systems.

I did not see anything in the OP that said anything about inputs driving coils. ????

I am guessing that based on this.

If I run 24VDC the current required to close the solenoids will be lower then if I go with 12VDC and I think that would be more important.

 

[don_resqcapt19]

Bob,

One issue with 12VDC is that it is quite possible on long runs that an input will not function properly due to the capacitance on the line. it takes hundreds or thousands of feet to see this but it can and does happen. The only real answer is higher voltages.

That also happens with 120 volt AC control circuits. The capacitance on long stop circuits can prevent them from working.
Don

 

[tom baker]

I see a lot of PLC systems now specifed as 24 VDC for operator safety. The systems I am designing now are 24 VDC, if we need to operator a contactor, then an interpoise relay is used.
However I have not seen (not saying they are not available) a PLC in 12 VDC
From a safety point there is no difference between a 12 vdc and 24 vdc system, as both will be a NEC Art 725 Class 2 circuit. A class 2 circuit limits the voltage and power for fire and shock protection.

 

[petersonra]

Bob,

That also happens with 120 volt AC control circuits. The capacitance on long stop circuits can prevent them from working.
Don

Thats true, but the "long" distance for a 12 V circuit where this can come into play is a lot shorter than for 24V or 120V.

 

[petersonra]

I see a lot of PLC systems now specifed as 24 VDC for operator safety. The systems I am designing now are 24 VDC, if we need to operator a contactor, then an interpoise relay is used.
However I have not seen (not saying they are not available) a PLC in 12 VDC
From a safety point there is no difference between a 12 vdc and 24 vdc system, as both will be a NEC Art 725 Class 2 circuit. A class 2 circuit limits the voltage and power for fire and shock protection.

Just because it is a class 2 circuit does not mean it is "safe". I think there is little doubt that a 12V circuit is inherently safer than a 24V circuit. The real question is how safe is safe enough.

I am not a big fan of 12V stuff because it might encourage people to use equipment that is really designed for use inside vehicles.

For some reason that I cannot quite put my finger on, there is a fair amount of interest in 12V systems. I think some of it is driven by solar powered stuff that uses 12V batteries for backup power in remote locations. I run across it from time to time. With the use of wireless networks and solar/battery powered equipment, it has become practical to put monitoring equipment in remote areas without the expense of having to run wires to it, which often has made such a thing impractical in the past.

 

[Entertron]

Leslie,

12 VDC PLCs do exist. We are actually one of the few manufacturers that actually offer this operating voltage.

Much will depend on your number and type of I/O that will determine the series.

Information can be viewed at www.entertron.com

Hope this helps.

God Bless,

Stephen Luft

 

[i2rservices]

12 v vs 24v

12 v vs 24v

There really isn't much difference as far as a "shock" hazzard for most people. I know people who can easily touch even 48vdc without feeling anything. Of course you don't want to get anything metallic across it, as you might expect. The 24v systems are much more common I think, as well as you're correct that the current would be about half as much. I'm sure you'll have everything grounded properly anyway. It sounds to me that this guy just likes 12v systems, or maybe he knows someone who "sells" them locally?...ha ha

 

[petersonra]

There really isn't much difference as far as a "shock" hazzard for most people. I know people who can easily touch even 48vdc without feeling anything. Of course you don't want to get anything metallic across it, as you might expect. The 24v systems are much more common I think, as well as you're correct that the current would be about half as much. I'm sure you'll have everything grounded properly anyway. It sounds to me that this guy just likes 12v systems, or maybe he knows someone who "sells" them locally?...ha ha

I am guessing there is some non-obvious reason that 12V makes some kind of sense for his specific application, like the customer demanded it.

 

[DaveTap]

Make everyone happy?

Make everyone happy?

Inspector seems to have problem with voltage potential to ground causing a shock hazard. So tell the health inspector that your equipment will only have a potential of 12v to ground... then get a power supply with +12v/-12v and connect your 24vdc controls... LOL... everyone's happy!!!

 

[petersonra]

Inspector seems to have problem with voltage potential to ground causing a shock hazard. So tell the health inspector that your equipment will only have a potential of 12v to ground... then get a power supply with +12v/-12v and connect your 24vdc controls... LOL... everyone's happy!!!

that is only true if it is a dual power supply with a common, and +/-12V, and the common is tied to ground.

 

[catchtwentytwo]

I don't know if this would help but we have a very complex data center power system that uses 48 VDC from station batteries for breaker operation. The controls use redundant 24 VDC supplies from two 48 VDC DC to 24 VDC converters from VICOR using Auctioneering Diodes

Perhaps you could find something that would let you run at a higher voltage (for voltage drop purposes) and still keep the AHJ happy.