Positive grounded 24VDC system.

[lquadros]

Hello,

We have systems that are 24VDC positive ground with negative signal as PLC inputs. Every once in a while we get this question as to why we are set up this way. No one knows the exact answer. However, we have gone so far along that all spare parts like PLC input cards, output cards, sensors etc. are stocked. It could be expensive and at times, confusing to change this system. Can some one point out the advantages or history of why a negative logic system could have been started? I am very interested to know.

Regards,
lquadros.

 

[Besoeker]

Hello,

We have systems that are 24VDC positive ground with negative signal as PLC inputs. Every once in a while we get this question as to why we are set up this way. No one knows the exact answer. However, we have gone so far along that all spare parts like PLC input cards, output cards, sensors etc. are stocked. It could be expensive and at times, confusing to change this system. Can some one point out the advantages or history of why a negative logic system could have been started? I am very interested to know.

Regards,
lquadros.

I don't know. We don't usually ground either side of the 24V supply.

Maybe it came from the automotive industry. It used to be a long while back that, in vehicle systems, the positive side of the battery was grounded. I think that it might have been something to do with contact wear in the distributor. Just an idle thought...

 

[pfalcon]

Corrosion.

Metal atoms from conductors can attempt to migrate from a positive conductor to a negative conductor.

If the negative is grounded then the positive wire may be degraded if the insulation leaks.

If the positive is grounded then the negative wire may be plated if the insulation leaks.

 

[Open Neutral]

Corrosion.

WhatHeSaid.

Telco has been that way since Watson's era.

 

[fmtjfw]

academic explanation

academic explanation

WhatHeSaid.

Telco has been that way since Watson's era.

http://lyle.smu.edu/~levine/ee8320/positiveground.pdf
See part 3 for more information.

 

[broadgage]

Yes it probably goes back to vehicle electrical practice.
Virtually all modern vehicles have the negative of the battery grounded, however in years gone by positive ground was the norm for vehicles and was probably copied for other low voltage DC systems.
Telecoms equipment normally has the positive grounded.
Off grid homes usually ground the negative of the battery bank.
DC electric railways, tramways and the like ground the negative.

Opinions differ as to the relative merits of positive or negative grounded systems.
Modern practice is to ground the negative unless there is a good reason to do otherwise, such as compatibility with existing equipment.

Back in the good old days of DC utility service it was usuall to use three wire DC systems with a center ground.
Negative to earth faults on such systems tended to outnumber positive to earth faults, it was said that the negative conductors tended to attract dirt and damp which would eventually lead to insulation breakdown.

 

[Rampage_Rick]

As mentioned, it's been the usual practice for phone lines going back to the plugboard operators. "Ring" is -48V battery, and "tip" is ground. It's one of the few places where red is the negative conductor...

Regarding the corrosion aspect, many pipelines utilize cathodic protection, whereby the negative terminal of a 50VDC 10-50A power supply is connected to the pipe, and the positive anode is grounded. Such a system will actually cause the pipe thickness to grow over time (rather than dissove away if the pipe were positively charged)

 

[jim dungar]

Bonding in power systems (e.g. vehicles and phone companies) can be done for different reasons than in PLC/control systems.

For PLC inputs, there is a peception of safety for different systems - is it safer to run with a faulted input or is it safer to shut down the entire process on a sngle fault.

The decision of grounding the positive or the negative often revolves around the performance of the electronics of the inputs and outputs (NP, PN, NPN, or PNP). The most common 'unswitched' leg is often what gets grounded.