24 Volt Inputs and Grounds

I just left a plant, with AB and Omron equipment. Company standard is 24 volt input. So far so good.

What I didn't care for, is the possibility that the whole plant is set up, is to ground the inputs for operation. While it doesn't happen TOO often, a wire accidently grounded cuasing a machine operation to function.

Did a forum search, but didn't readily find anything that help.

My thought is, supply +24 VDC to input for operation, ground the -24 vdc.

Sorry guys, gonna do a poll on this one...

regards.....casey

We've discussed this topic before (LINK1 and LINK2), but it can't hurt to get some input from the newer members. I voted for PNP with a floating supply, but I'm not against a grounded supply, as long as it's the negative side that gets grounded.

I still see lots of US-built stuff with negative logic...

beerchug

-Eric

Definately positive supply to inputs. I do not ground the negative as I use switchmode power supplies and, usually, grounding the negative means loss of isolation.

The Japanese also like their negative inputs. They call it NPN inputs. Opposite to AB I believe.

No problem with sinking outputs though. They are normally just going to relays etc in the control panel.

Most European companies appear to use positive input and sinking output.

signal "on" should always be different from ground level and
accidental short to ground should be detected (blown fuse will also
get attention) but it shall not cause unwanted motion.
while you can make circuit that is oposite and sure it can work,
in industrial enviroment it is dangerous and not good.
failing to do so (for example using npn I/O when negative side of psu
is grounded) is more than unprofessional - it is reckless and can send
you to court and possibly jail if someone does get hurt.
as you have noticed with the inputs japanese like to wire inputs to negative rail
and outputs are sinking as well and positive end of psu is grounded.
makes no difference except that you need separate power supply for
your safety circuit. some of the safety products have been obsoleted
because of ground terminal which is internally connected to negative
power terminal (example G9S-301 etc.).

What did you used to teach, or was it preach? Preach, or was it dictate?

mb

m

Dictated? I "emphatically stated" that all secondaries were to be grounded, PERIOD!

Power supplies were a different issue, specially when a machine may have several for what ever reason, (current capacity, seperated controll cabinets, multiple MCR's, or?). Sometimes really strange ground looops develop with PS's, and sometimes PS output voltages that are slightly different can act up.

Generally, I like to ground the PS, but ALWAYS the CPT secondary.

Dictate? Behave or I'll tell everyone that you are really in a fifth wheel! Behave or you'll have to take State Logic over. Behave or you'll have to stay after.....oh nevermind.

kc

I've seen mostly 24 volts with positive to inputs, and no grounds. Heard about grounding the negative supply, but I think it all depends on the situations and individual requirements, am I correct to say so?

regards
Sherine T.

I don't ground anything from the PS and use +24 to my input cards.
Rational for not grouning? Its simple, we're an OEM in a really loose (electrically) industry. I can't count the number of times I've seen my machines installed with no ground of any kind, just 3 phase to my disconnect... Ground loops are common for us, in fact we've got one product with 5 24VDC power supplies just to keep the ground loops to a minimum.

Inputs

I would like to see all the power supplies float the common. In all the signal applications (analog) I will isolate the common. However both Allen Bradley and Modicon (dont know about others) reference their DC inputs to the module common. If you isolate the power supply and ground the PLC as required by their Installation manuals, you have effectivly grounded the supply. Most electrical codes require all metal non current conductors be bonded and grounded to the system ground. Even isolated grounds are grounded at the system ground point. Power supplies can have isolated grounds, but must still be grounded as per code. You should have seen the fun we had installing an ABB electric locomotive rectifier station underground. The trolly line was negative and the track was positive. Europe has positive ground. The track "floated" and bondind was very important. I believe Ohio Brass rectifiers can be hooked up either way. When you import PLCs you have to read carefully how their inputs work.
Yes... I learned this the hard way many years ago...(let the smoke out the PLC card)..Such fun!

Bruce...

Hmmmmmmm

You need another answer in your poll.....

O What the customer specifies.



Dave

panic mode said:

as you have noticed with the inputs japanese like to wire inputs to negative rail
and outputs are sinking as well and positive end of psu is grounded.

Click to expand...

Yes, in Japan it is conventional to use sourcing inputs and sinking outputs, but I've never seen a grounded positive or negative on the power supply.

Since I've been working here, I've come across hardly any problems due to noise on analogue circuits, probably because the 24V is floating. If fact the only noise problems I've come across recently have been on European built panels.

My pet hate is when panel builders assume that ground is the same as the 0V of the DC supply (i.e. using ground as part of the normal control circuit).

A few weeks ago I came across a panel with two 24V DC supplies, not connected in parallel and the 0V sides of both supplies were grounded. The panel builder had used one power supply to power a transistor output module (source type) but the load for each output used the 0V of the other power supply. i.e. ground was being used as part of the control circuit. Guess what happened when a service engineer replaced one of the power supplies and forgot to reconnect the 0V ground properly...

...The transistor outputs stuck wide open.

I always prefer to ground p/s negative. We have a lot of analog inputs, as long as the shields were only bonded on one end, and have never seen a big problem. The advantage to having the negative grounded, at least in our plant, is that there is not always a DC COM in the junction box or panel you are troubleshooting 24V+ wiring. In someplaces, you would need a very long piece of wire to get COM.

I've never liked grounding the negative of the PS, I'd rather have an isolated system.

But, we're all slaves to what the customer states in his specs.