DC Power supply with integrated ground fault monitoring? 1

[rmt131]

Hello,

I was wondering if anyone knows where I could find a mid-sized (about 30A, 750W) 120VAC to 24VDC power supply that has integrated ground fault monitoring of it's floating DC output with remote indication? Din-rail mountable would be a bonus, but not necessarily required...

I can find 24V DC power supplies, and I can find stand-alone ground fault monitoring relays for floating DC systems, but I can't seem to find one product that does both...and cabinet space is an issue...

Thanks for reading,
rmt131

 

[itsmoked]

I could never imagine you finding an off-the-shelf version of one of those. Never in a million years...
(Now that should spur someone to provide an example!)

You better start working on alternatives.

Another question would be why on earth you would want one? Doesn't make any sense to me at all. Power supply outputs are generally designed to have outputs isolated mechanically from the inputs. This means that the outputs are floated and the user is free to tie one of the outputs to whatever ground plane is desired. So how/why would a PS maker ever provide some form of ground fault detection on this isolated output? (They don't even know which one the user would want to use as the ground.)





Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rmt131]

Thanks itsmoked, for throwing down the metaphorical gauntlet! If that doesn't get responses, nothing will!

As to why we need this: we leave both poles of the DC supply floating when we serve 24VDC power to our PLC's and end devices (sensors, instrumentation, etc.). This has two benefits that we capitalize on:

1. if a technician accidentally touches a pipe or other grounded metal object with a wire while changing out an end device, there are no sparks generated because there is no return path through ground.

2. Similarly, if a ground fault does develop in the field, as long as it is confined to one pole (i.e., pos or neg, not both), it doesn't shut down the system. As long as we can monitor the DC near the point of supply for ground fault on either pole (a simple high-resistance balanced voltage bridge works well - see Startco's SE-601 at

http://www.startco.ca/se601.html),

we'll know if a wiring problem is developing in our downstream devices...before things go 'pop'. Detecting which device it is is sometimes fun, but at least we know it exists in time to do something about it...kind of like having an NGR on your DC system...

So there you have it. I'm not sure if anyone else sets up their DC system that way, but this is the way we've done it for quite a while now. It seems to be working for us...but you're right, if it's a rare sort of application, chances are there's no such beast as the one I'm hunting (for the most part, our existing ground fault detection is integrated right in the UPS, but that won't cover this particular supply I'm afraid...)

Oh well, I guess we'll have to scrape out some room in our cabinets for the Startco relays...unless someone wants to surprise me? Please?

thanks again,
rmt131

 

[stevenal]

Hindle makes chargers with ground fault monitoring.

 

[itsmoked]

Well thanks for the update. Your scheme makes some interesting sense. Generally it isn't done that way that I have seen. How many 'end devices' and PLCs do you guys jack onto a single supply anyway?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rmt131]

Thanks for the pointer, stevenal!

I'm checking out these Hindle chargers now...they're pretty slick! They are a bit bigger than I was hoping for (I was angling for something more the size of Phoenix Contact's DIN-rail mountable Quint supplies), but hey, they do integrated GF monitoring and alarms...

They are a little battery-charging-specific for my application, however(float/equalize controls, etc.) I don't think I mentioned it above, but I be needing these to power a PLC, some attached instrumentation and sensors, and a small DC motor. Cost is a factor (isn't it always?), and although we don't want to go to a bigger cabinet if we don't have to, we can probably do that and pop in the Startco relays quite a bit cheaper than going with the full float/equalize capable charger units, each with their own cabinet and associated hardware... Still, thanks so much for pointing them out, I will definitely keep them in mind next time we're looking for a remote UPS system replacement...and it's nice just to know such equipment is out there!

thx again,
rmt131

 

[rmt131]

Hi itsmoked,

It's not so much 'how many devices per supply', since we usually only have one big DC supply, the site UPS. This gets divvied up in the DC panel, which then provides individual DC supplies to local PLC chassis with their associated I/O.

AFAIK we usually don't have more than one or two chassis on one breaker, 5-12 assorted cards per chassis and each PLC might use 10-30 end devices (transmitters, solenoids, etc.) Is that what you meant? I'm afraid automation is not my area, I usually just give the automation guys the power they ask for, in the proper flavour and wherever they need it...sorry if I'm mixing something up here...

thanks!
rmt131

 

[itsmoked]

Yeah that was all I was asking. Just stocking the memory bank, thank you.

Seems to me, the wiring quality at a site shouldn't be so lax as to require detection of shoddy work but if you have a truck load of points then the probability of a short-to-ground of course goes up. I guess 30 or so points makes sense for your case.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com