Please suggest what best I can do to blow the fuse?

[drspanda]

Dear All experts,

The system:
I am having AB PLC to operate and control coal handling plant with many conveyors, flapgates, vibrating feeders etc.,

Power supply:

All I/O's are of 24Vdc...For Inputs +24 V DC gets distributed to Input devices via fuses (200ma each for each input)(like start Push button, Limit switches, Pullcord and beltsway....)

-24V DC Negative is grounded at panel.

The Problem:

Suppose one wire gets grounded at beltsway switch no.5 (for a single conveyor there may be 10 such switches connected in series and All together constitute a single input to PLC)....Fuse should blow up BUT NOT HAPPENING.

But if another wire of say Pullcord no.10 gets grounded We are facing a safety related problem because operation of Pullcord no 1 to pullcord 9 will not result any change in PLC INPUT BECAUSE +24V DC is available at Pullcord 10 due to ground.

My Attempt:
I tried to measure current flowing thru that particular fuse (where there is a ground) and found it is less that 100ma...

PLC panel is about 1000m far from the I/O.

Any suggestions??

 

[bernie_carlton]

You say that -24VDC is grounded at the panel. Are the panel and all the switches bonded to the same safe ground?

 

[JesperMP]

Use a safety relay to cut power to the belts. Dont use the PLC for this unless it is a safety PLC (which I suspect it is not).

1000 m is a long way, so you have 2000 m all in all.
I am not so surprised that the fuse does not blow, and that the PLC input is still on is probably because a 24V input can often operate down to 10V or 15V.

If you had used a safety relay, then its internal current limiting would have meant that it would have tripped.

At 2000 m, be sure to get one of the latest generation safety relays.
The newest ones from PILZ for example only draw 5 mA. I had problems in the old days with some old safety relays that operated at 500 m.

 

[Bob A.]

I'm not sure that I completely understand. The normal state for a "Stop" circuit is for the PLC Input to be in a Logic High State. Any loss of signal would cause a "Stop". This is a critical saftey requirement such that problems in the Stop Circuit, tend to "Fail Safe". It sounds to me that you are using a "Stop" circuit that can be inoperative and the system still be in a "Run Mode". If this is the case, I strongly recommend that you reverse the sense of the circuit logic and fix the wiring problems.

In regard to the fuse not blowing, is it possible that the circuit resistance is so high that a short in the system cannot pull enough current to blow the fuse.

Considering Ohms Law, 24 VDC / .2 Amps = 120 Ohms

I suggest that you make some resistance measurements. Your wiring maybe too small combined with the distances that you site. A normal 18 AWG copper signal wire is about 22 Ohms / KM so the most that you should see is about 50 to 60 Ohms allowing for some contact resistance to creep in somewhere. If you are only seeing .1 Amps, that implies that your resistance is 24 / .1 = 240 Ohms.

I would review these findings with the electrical engineer that laid out this system.


Best Regards,

Bob A.

 

[Erik Frenks]

I think You have 2 main problems:
1. The fuse is not blowing out: If the resistance of one cablelength and the earth circuit is to much for the value of the Fuse, You have to decrease the Fuse value (If possible) or replace the cable ($$$).
2. You Inputs are not always as they should be (H or L): Increase the 'Load' of the circuit (And Power supply!) by placing an relay near the PLC and use the relay-contact in your PLC-input.

This is only for normal switches, NOT for Safety-circuits like a Pullcord! See Jesper's reply

 

[TConnolly]

Some 24V power supplies, in particular switching power supplies, act very quickly to shut down the 24V power in the event of a short circuit - quicker than a fuse will blow. Check the specifications of the power supply.

That said, it sounds like you need to reassess the design of your safety system.

 

[mordred]

But if another wire of say Pullcord no.10 gets grounded We are facing a safety related problem because operation of Pullcord no 1 to pullcord 9 will not result any change in PLC INPUT BECAUSE +24V DC is available at Pullcord 10 due to ground.

As Bob.A mentioned this is definetely wrong

the fact that this can happen suggests that the PLC is not a safety PLC and whoever designed the safety circuit did not follow the regulations governing the use of machine safeties Is your inputs all 0 volt activate???

The other question is what safety classification does this machine fall into for example a class 3 safety circuit uses multiple contacts per safety device to detect welded contacts via another contact. Or positive break contacts that break those welded bonds

 

[OkiePC]

1st, I agree with all the coments on safety above and...A pull cord stop sounds like a safety device, if so, it must function as one in hardwiring...

For DC at those distances?

My Attempt:
I tried to measure current flowing thru that particular fuse (where there is a ground) and found it is less that 100ma...

PLC panel is about 1000m far from the I/O.

Any suggestions??

If it is less than 100ma, then it is not a very good ground, unless the power supply is "crowbarred" and shutting down to a minimum voltage before the fuse can blow. Better grounding and faster fuses may help.

If posible, though, I would suggest that you use remote power supply(ies) at great distances from the PLC sending AC to supply them for the long runs. Then fuse the (now much shorter) DC wiring appropriately at each remote supply.