Question about fusing in safety circuit.

[CircuitNewb]

Hi all,


I'm working on a safety circuit and had some question about fusing.


Incoming supply - 120V/15A
Power supply - PSL-24-060 https://cdn.automationdirect.com/static/manuals/psxinstall/PSL-24-060.pdf


Safety relay - Pilzs S4 + S7 expansion module
VFD - Inverteck P2 with STO


Supply -> 24V power supply -> safety relay -> VFD


My question i was wondering is if i need to fuse/cb betweem each component here?


for example Supply -> 24v PSW -> (fuse/cb) -> safety relay - (fuse/cb on the outputs to STO)


The power supply states that the max inrush current it can handle is 30amps and has an internal fuse (t3.15 amps/250VAc) the power supply manual states I should use a minimum value of 16A B- or 8 A C- characteristic but unsure of what amperage to use?


for the fuse between the power supply and safety relay the pilzs s4 shows a max inrush current of .5 amps, but the expansion module lists a a maximum impulse of 2.7 amps. how do i size this fuse/cb?


Lastly for the safety outputs - these will be connected to the STO terminals of some VFD's and the Inverteck P2 drive specifies a switching current of 100mA. Do i need any protection on the output side as well ?


Hope this makes sense, I am new to safety circuits and circuits in general.


Thanks.

 

[Robb B]

I think your power supply manual means MAXIMUM fuse size of 16A or 8A. You should, in general, try to size your fuse to the smallest possible that lets the circuit still operate. Take into account loads like transformers and motors that may have an inrush current and require a time delay fuse or similar. I would fuse the output of the power supply, but not the inputs to the VFD. If you have a fuse upstream of the safety relay, you shouldn't need one downstream. In your case, I would probably use a 2.5A time delay fuse, but there might be other particulars I don't know about (indicating lights? other relays?).

I'm not sure, but your last question sounds like the drive itself requires 100mA to trigger the switch, if you were really protective of the drive, you could put some small fuses on those inputs (notwithstanding my previous comment) but it usually isn't done. It also makes troubleshooting way more difficult if one of them blows and no one knows there's fuses on the inputs.

 

[joseph_e2]

Normally, I would not fuse the wires from the safety relay to the drive's STO terminals. But....if the drive were in a different enclosure and you were worried about the wires being grounded by damage, then it may make sense. Just be sure to document it very clearly because it's not normally done (in my experience).

 

[CircuitNewb]

Thank you Robb and joseph,


The drives are in a different location, but im not really concerned about cable damage ( they will be run neatly and in their own cable trays)


I will likely fuse/cb the output of the power supply - thanks Robb.


Would you suggest also the 16A or 8A fuse between the 120v/15a supply and the 24V power supply ?


Regards,

 

[Robb B]

The manual does say minimum 16A and 8A, but those look like they're for circuit breakers, and likely for the 120v and 240v ratings on the inputs. The power supply claims to have internal fuses and has been tested on circuits up to 20A. If you want to fuse the input in addition to the internal fuses, you could but it is unnecessary, I'd probably go no bigger than 0.5A. The manual also says maximum output power at 24V is 2.5A, so I'd put fuses no greater than that in the output.

 

[lendpack]

The manual does say minimum 16A and 8A, but those look like they're for circuit breakers, and likely for the 120v and 240v ratings on the inputs. The power supply claims to have internal fuses and has been tested on circuits up to 20A. If you want to fuse the input in addition to the internal fuses, you could but it is unnecessary, I'd probably go no bigger than 0.5A. The manual also says maximum output power at 24V is 2.5A, so I'd put fuses no greater than that in the output.

Thanks for your suggestion, I will try!