Shorting 24vdc external power supply

[alizawi]

Hi, quick question. First of all here is the scenario:
I have an external 24vdc power supply for a DI module on a certain PLC. The common side of the supply is grounded. There is a common fuse in the module to which the 24vdc is connected to. I have two pushbuttons connected to the inputs of the module with slow blow fuses between the module and each pushbutton. If i short circuit the external power supply, what should happen to the fuses between the pushbuttons and the DI module? How about the main fuse?

 

[nakulak]

diagram would be helpful

 

[micromind]

Generally speaking, in the event of a short circuit in a system like this, a fast-acting fuse will blow first. In the absence of that, a slow-blow fuse will blow. In the event of no fusing at all, most switching type power supplies have some sort of short circuit protection built into them, usually auto-recovery.

Since there's no 'start surge' associated with DC circuitry like this, fast-acting fuses are the best choice. In the absence of any other protection, a digital output card of a PLC will usually burn up before a slow-blow fuse opens the circuit.

 

[petersonra]

Hi, quick question. First of all here is the scenario:
I have an external 24vdc power supply for a DI module on a certain PLC. The common side of the supply is grounded. There is a common fuse in the module to which the 24vdc is connected to. I have two pushbuttons connected to the inputs of the module with slow blow fuses between the module and each pushbutton. If i short circuit the external power supply, what should happen to the fuses between the pushbuttons and the DI module? How about the main fuse?

Unless I am not understanding your setup, my guess is none of the fuses blows, except maybe the main one, and that is iffy. Many power supplies will just shut off under a SC condition before the fuse can blow.

 

[GeorgeB]

Hi, quick question. First of all here is the scenario:
I have an external 24vdc power supply for a DI module on a certain PLC. The common side of the supply is grounded. There is a common fuse in the module to which the 24vdc is connected to. I have two pushbuttons connected to the inputs of the module with slow blow fuses between the module and each pushbutton. If i short circuit the external power supply, what should happen to the fuses between the pushbuttons and the DI module? How about the main fuse?

It sounds like you have a "true low" input module from your description, and shorting the inputs to ground is the "on" mode. Losing the 24V fuse on the module could cause all inputs to appear true depending on internal logic.

I've never seen an input module with fuses. I've never seen a control system design with fused inputs. I'd research this in the manual. If it doesn't answer the question, I'd research it by testing. "main fuse" is to open-ended for me to have any idea to what you are referring.

 

[alizawi]

Thanks for the answers. I might have not been so clear. The module I am specifically dealing with is SDV144 of Yokogawa prosafe. You can see details at http://aspzone.us.yokogawa.com/downloads/dcs/prosafe/gs/GS32S06J10-01E_001.pdf
If you look at the drawing on page 8 (4 of 9in the pdf file), my normal fuse (rated 0.3A) is between the module and 24vdc. The slow blow fuse is between DCN and the module (should this be a fast blow fuse?). So what will happen to my slow blow? Btw, common is grounded.

 

[petersonra]

what is DCN?

so we know just what you are asking, maybe a sketch is in order.

btw, for those w/o a lot of control experience, it is not uncommon to fuse every all I/O points and the coltage feed to a module. Its done so you can isolate things for debugging a smuch as any other reason.

It is also hoped that if you do have a ground fault it will affect fewer parts of the system.

 

[alizawi]

Ok, to be more clear, please see the attached image. The fuses for the module channels are FAST BLOW. now the new senario at the field is that grounding was done on the field side (between the PB and the fuse of DC10). Which fuse would tou expect to blow out first. The case here was that fuse "A" blew out. Any ideas?

 

[alizawi]

No one has any ideas?

 

[ELA]

If fuse A is slow blow and Fuse B is fast I would expect Fuse B to blow first , given your short to ground at the field side.

All fuses have trip time curves and the fuse selection may not have been coordinated properly.

In addition Fuse A may have been weakend from repeated slight overloads.

 

[LHarrington]

Good morning,
Is your DI sinking or sourcing? Sourcing should blow fuses. Sinking would be normal condition for input and it wouldn't blow the fuse.

LHarrington

 

[Controls]

I guess, I like to know the reason behind this question. What is the concern here ?. I reviewed your pdf on page 8, I saw your fuse on your common 24V supply that is for the all DI module. Do you also have dedicated fuses on your PBs.Why was that necessary ?. If it is very critical to protect the main supply fuse for the all module with this application, what was not design to have indivual fuses for each inputs on the module itself ? Sorry, I am just trying to help. Let me know if am on the wrong track.

 

[Paul B]

I don't think "B" will ever blow

 

[alizawi]

Yes, there are fuses just as shown in the drawing. A colleague and I were wondering why there are fuses for each input. Could be for testing only. Anyways, what happened while shorting from the field side was that fuse A blew. That's why we were confused. Why would fuse B never blow? Did the current reach such a high level on the 24V side that it blew out fuse A and that was that?

 

[petersonra]

Yes, there are fuses just as shown in the drawing. A colleague and I were wondering why there are fuses for each input. Could be for testing only. Anyways, what happened while shorting from the field side was that fuse A blew. That's why we were confused. Why would fuse B never blow? Did the current reach such a high level on the 24V side that it blew out fuse A and that was that?

Most likely the individual input fuses (called "B" on your sketch) are simply there to isolate that circuit. You might find it handy to be able to disconenct an individual cicruit if you need to work on it.

Fuse A is there to protect the field wiring.

If fuse "A" is slo-blo and fuse "B" is fast blow one might expect a short to ground downstream of fuse "B" would likely blow fuse "B" first, thus preventing fuse "A" from blowing. However, you would need to look at the curves for the fuses to find out just what may be going on. Sometimes with high currents (such as might happen with a short) the curve may show the slow blow fuse blowing before the fast-blow.

There is no real definition of fast versus slow blow fuses, especially with the small glass fuses people tend to use for these applications.

 

[alizawi]

Most likely the individual input fuses (called "B" on your sketch) are simply there to isolate that circuit. You might find it handy to be able to disconenct an individual cicruit if you need to work on it.

Fuse A is there to protect the field wiring.

If fuse "A" is slo-blo and fuse "B" is fast blow one might expect a short to ground downstream of fuse "B" would likely blow fuse "B" first, thus preventing fuse "A" from blowing. However, you would need to look at the curves for the fuses to find out just what may be going on. Sometimes with high currents (such as might happen with a short) the curve may show the slow blow fuse blowing before the fast-blow.

There is no real definition of fast versus slow blow fuses, especially with the small glass fuses people tend to use for these applications.

Your explanation makes sense; I agree. Thanks.