VERY wet conditions 24 VAC control ckts vs 120 VAC ??

Ok so where I now work is food industry. Workrooms are chilled so we got condensation. Water all over and enthusiastic cleanup employees with water hoses
- if it is there HOSE IT DOWN
these guys would make good firemen.

Equipment (mostly motors) voltages commonly are 120 VAC single phase and 90 or 180 VDC and for three phase 208 240 and 480.

My main concern is the control circuits many of which are 240.
I know this is no longer allowed by NFPA 79 and even if that were not the case I don't like "hi voltage" control. SO I gotta start advocating 120 VAC control voltage or less for AC equipment at least. DC motors from what I have seen (so far) are all driven with solid state controllers.
In my mind 120 VAC motors should be on GFCI - that will be separate battle.

SO for 240 VAC or higher

IT seems to me with all this water and even with an isolation transformer (properly fused grounded etc) for 120 VAC control I should also use a GFCI. That way if something goes wrong the GFCI will trip and we do not have runaway equipment or electrocuted employee.

Just the control transformer may be enough battle.

IF I use 24 VDC or 24 VAC then I would not have to go thru the battle for the GFCI.

Have not decided which just yet probably AC since more equipment is built for 24 VAC than for 24 VDC.

Am I thinking straight
- any good devil advocate advice
or better ideas?

Thanks

Dan Bentler

Use 24vdc It is safe, use SELV (safety extra low voltage) isolated power supplies. WIth 24v ac you can get problems with coil switching I only use 24vac where the size of the circuit doesn't justify a dc power supply but as I only pay about £20 for a 1.3A and £60 for a 5A 24vdc power supply then that is almost never.

Switching large contactors can be a bit problematic with dc controls but the new electronic controlled ones have alleviated this to some extent.

Thanks for help John

Dan Bentler

further advise for high washdown where possible have all junction boxes etc above the equipment to be washed this will cut down on the amount of damage. look into placing heaters into your control cabinets to help remove the condensation. heat lights works good. make sure all wiring is fed from the bottom not the sides or top of the cabinets. Avoid alluminum wiring and ground lugs use copper the caustic soda associated with cleanup will corrode aluminum at a high rate.

Mordred

For some reason boss does not want to put cabinets up high. Likes em nice n low (they are out of the way there,,, but). I do have some challenges here.

Thanks for mentioning aluminum. That had not crossed my mind yet.

Dan Bentler

Tell your boss that safety of the employees and machinery should always come before personal preference when it comes to location of electrical equipment.

That certainly will present some challenges for the cabinets we were not able to move we installed splashguards the main concern is the PSI rating of the washdown hoses our plant operates our washdown at 180 psi witha max orifice size on the nozzles at 3/16 inch. The splashgaurds do offer some protection but one thing I've long discovered is you never will keep the water out so remove it as fast as possible. In your case you can use the heaters for that or another method is to add air to the cabinet to give it positive pressure have a pressure relief valve so you don't over pressure the cabinet. Its probably cheaper to use the terminal mini heaters with a terminal thermostat. From experience your biggest type of electrical faults is going to be shorts to ground I regularly check the ground to neutral voltages any time it exceeds 1 volt feedback I have the circuit looked at in most cases it will indicate moisture build up a junction box somewhere. Hope this helps.

Tharon,

My primary reason for opening up these cans of worms is employee safety.
I got out of health and safety work because I got tired of arguing with people. I really don't care if they want to do good electrical practice to keep their equipment running after all it is just machinery. What I DO care about is making sure no one gets hurt TRIPLY so on any wiring work I do. I have done my share of fatality investigations and I do NOT need to do any more. My brothers call me the king of overkill and that is true BUT so far none of my wiring work has hurt anyone nor have there been any fires. I intend to keep it that way.

My boss now has two assisting him and looking for 3rd. 3 years ago he kept this place running all by himself. To make a long story short he took a few shortcuts (a bunch) - I got my work cut out for me. He is one of the better jury rig / bubble gum and baling wire artists I have seen for quite a while.

Dan Bentler

Another point is food quality by having the wiring below the product you avoid food contamination and that may be the reason why your manager may want to keep the electrical below, rather than above the working tables.

I worked in a meat packing for over 7 years. I found it best to use 120 vac control it stands up to the water the best. Places where 24 v was used I was replacing components almost everyday.

Also found that it is impossible to keep water and condensation out of control panels the best thing I found was to give the water a way out of the cabinet by drilling a few small holes in the bottom.

Good luck

John

Thats funny in a way I found that the 24 volt was more reliable and yes we do us the holes in the bottom of the junction boxes for provided there wasn't any sensitive electronics in that cabinet or junction boxes.

Thanks for the help guys.
So it appears that some like 120 and others like 24 volt control.
Dang now I gotta make the choice. Think I will try and go 24
will have to double check with guy who supplies relays etc and see which supports more 24 AC or DC.
My hunch is AC so the control voltage for now is 24 AC.

Dan

In dairy factories here in NZ (there's a lot of them & they're huge), control voltage is 24. 24VAC was the norm about 20 years ago, but there seems to be a migration to 24VDC for new work.

It may be influenced by the availability of higher density I/O modules for 24VDC vs 24VAC.

Having worked more than 10 years in the food industry, including specifying and engineering projects for some of the bigger food MFG companies, I can say there are a number of things to look out for.

For control panels, if possible locate them outside the wet areas. Normally there would be dry switchroom in which Motor Control Panels and PLC panels lived.

Keep terminations in the wet areas to a minimum. I would run multi-core cables from the switchrooms to the field (no trunking or conduits to fill with water). This has been made easier lately with bus networks like Profibus and Asi.

Where you have local termination boxes, they should be made of toughened plastic or stainless steel and rated at least IP65. Not sure what the US equivalent is, but this site shows what I mean by the IP rating http://www.memonline.com/guide01.html.

Any holes in boxes should be at the bottom and as mentioned nby others, where particular problems occur.

Sometimes holes are put in the bottom to ensure any water ingress comes out, be careful here though that water splashing up does not enter the panels.

24VDC has been used almost universally for control, safer around water for a start and Asi enforces this even more.

One of the worse I've seen in recent years was a chilled wet room for pasta salad pot filling, where they actually had 415VAC bus bar running through the building to supply the machines. They washed down with high pressure hoses no less, criminal.