24VDC power supply fuse protection

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Thread Starter

Sakram

In phoenix 24VDC power supply, (-) is isolated from GND, in this reference my understanding is that when (+) gets accidentally in touch with ground the fuse will not blow off. So having fuse on the (+) connecting to the Digital I/p module is not required. Please confirm.

But instead of getting +12V across (+) & GND, and (-12V) across (-) and GND, we have noticed +4.5V and -20.5V respectively. Please let me know the reason for this shifting.
 
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Bob Peterson

To your first question - required can mean a lot of things. It might be a local government enforced electrical standard or plant specification could "require" it. It may well work just fine without the fuse. The power supply might have built in overcurrent and short circuit protection so it may not need an external fuse. I like grounded DC supplies because it simplifies troubleshooting. It is also required by UL508a.

To your second question, if it is isolated from ground there is no ground reference, so the measurement is pretty much meaningless.
 
S
It's bad practice to run your 24VDC ungrounded. I've seen it cause not only inexplicable bizarre operation issues, but even damage to control system components. So ground it.

As far as overcurrent protection goes, that will depend on your supply. A lot of current (no pun intended) supplies internally current limit, so between that and the protection on the AC line, I usually don't bother.

But....if you're unsure whether or not it's needed in your application, what's the downside of putting in a fuse or breaker? It's like $20. Do make sure to check DC ratings on the fuse or breaker, they're usually less than on AC due to no zero crossing.
 
R
I use branch CBs to isolate the 24 VDC from its load but that is not "protecting" much of anything. Any industrial rated PS has current limiting and can stand short circuits all day if that happens. The CBs are used to isolate logical parts of the circuitry (one to each I/O module, ethernet switch, etc.) If there is a short on one of these circuits, the CB trips and the rest of the 24 VDC loads continue to be powered.

The unbalanced voltage to ground is simply because of a slight variance of the high impedance to ground between the 2 wires. Since you don't have an intentional connection to ground, the reference could be anything in the 24 VDC range. It's possible that the impedance is a significant portion of the meter's impedance and the act of measuring the voltage to ground is changing the voltage of one side of the circuit. Even though a voltmeter has a very high impedance, you are placing it in parallel with the existing impedance and that can change the measured value.

I have occasionally worked on an ungrounded control circuit but they are few are far between. I would not have one intentionally unless you are addressing that small group of situations where it is called for.

Russ K
 
Dear Bob,

Thanks for your reply.

We are extending 24VDC(+) from the PSU to the limit switches in the field w/o fuse in the (+) path and returning the (+) from the NO/NC of the limit sw back to DCS DI module . (-) reference is directly connected to the DCS DI module from the 24VDC PSU.

I understood from your message that there is no requirement of fuse on the path of (+) connecting to the DCS DI module individual channel even in case (+) accidentally get in contact with ground unless until (-) is grounded.
 
Dear Russ,
Thanks for your reply.

We are extending (+) to field limit switches and connecting the return path to the DI module of the DCS system to detect the status of Limit switch, without having fuse for the individual(+) supply extended to the field limit switch. In the case of grounding (-) to GND, then it is required to have fuse protection for the individual channel to prevent 24VDC PSU over current protection activation when (+) accidentally touches with the GND in the field,which could disturb the 24VDC supply extended to other circuits totally.

Please clarify.
 
R
I have seen power supplies ungrounded with a pair of lamps
Positive - lamp - ground - lamp - Negative.

The lamps glow at half Voltage until the wiring gets a short somewhere.
As a previous poster said it's difficult to troubleshoot.
There's no substitute for a grounded Negative with lots of fuses or even better circuit breakers to indicate a problem.
 
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