Interlocked Disconnect Required for Network Switch Dist Cabinet with 480VAC to 24VDC PS


Need some code guidance please regarding how to implement this for the lowest cost...

I want to place about a dozen 16-port Industrial Ethernet switches throughout our facility for a distributed control system. Each will be a zone fed by fiber. Then copper to the zone devices.

I don't want to run the switch off the 120 house power breaker panels. Someone plugs in 3-4 fans, a microwave, and two crock pots...pops a breaker... and I lose my network. This is what our IS/IT department for the in-plant Cisco gear. A few times per quarter their switch is offline because of a popped breaker due to shared circuits. I'm not sharing my important stuff with someone's blue tooth speaker and I'm not putting on breaker locks. Heck our panels are so full I don't think I could get a dedicated circuit.

So my plan is to run directly off a dedicated can from our bus duct to a 480VAC three phase to 24VDC output power supply. The power supply feeds the switch. I won't have to have a transformer. It would have a low parts count. Really simple. Like I can do that with a single three phase fuse holder and a ground lug sort of simple.

I'm trying to figure out if I need an interlocked door switch. I'm trying my best to get around it. I'm hunting through NFPA70 and 79. This isn't a motor controller (430). It's not really an industrial control panel (409). It doesn't really fit into the telcom areas of the code either. It's not a industrial machine (670? ---from memory). UL508A isn't much help either. I'm getting really frustrated.

I don't feel like this could be considered a junction box. While there aren't any controllers or switching devices in the box I think it would be a stretch to try to sell it to the AHJ under that premise.

Can someone please point me in the right direction as to what the minimum requirement would be such that I don't have to interlock the door with power off.

I'm also curious is anyone else has gone this route for the same reasons.

If I cannot have my way please give your recommendations for a good low cost disconnect that doesn't suck, wear out, become misaligned, or break after a year. ;)

Thank you for any direction.


Senior Member
First off stop. Do not even attempt 24 VDC power distribution. It is very limited in terms of distance.

Second the whole reason for the interlocking stuff is because some starter disconnects are not designed to open under a load so hence the interlocks to trip the contactor. A 60/30 A fused disconnect is however designed to be opened under load.

Third buy good true industrial switches such as RuggedCom or Hirschmann. Two things to look for is a lot of industrial equipment has absolute packet/second limits (AB drives 600 packets/sec) so need to be able to set maximum rates. Cisco switches don’t do this despite the advertising. Need at least passive if not active IGMP snooping for AB gear and goofy special switches for Siemens nonstandard Ethernet protocol. Also port lockout is a bug, not a feature. And any switch (Cisco again) that takes 20-30 minutes to boot is a nonstarter. Finally look for ring protocols, NOT RSTP and use it. That way you can take a switch down for maintenance or because someone plugged in 5 microwaved and survive it with no issues. Avoid UPS. UPS means changing batteries every 18-36 months,

tom baker

First Chief Moderator
Staff member
As far as your door switch goes, what voltage are you wanting to interlock, 480, 120, 24VDC?

For switches I would suggest Rockwell Automation Stratix models, under the hood they are Cisco which keeps the IT folks happy, and rockwell has used industrial electronics and they DIN rail mount with DC power supply (can have two). there are many versions of the stratix, layer 2 and layer three, nearly all are designed to use with SFPs. The catalog number is a 1783, this is a good, if detailed overview

Now as far as power goes, I agree no AC ups. We were using AB 1606-XLS power supples (AC to 24 VDC) 1606-XLS240UPS, a DC UPS and its companion battery, 160-6-XLBatAssy.

The DC UPS charges the 12 VDC battery, and then in a power outage converts the 12VDC to 24 VDC to run your equipment, Its far more efficient than a 120VAC UPS which is 120VAC>24VDC to battery>back to 120VAC, to equipment, power supply and then back to 24VDC. I rand some tests with a 500 VA UPS, 3 hours for a control panel, on UPS, 3 days on DC UPS,

The DC UPS has some nice features that monitors battery life, on batt, and ready.

Now the battery is claimed to have a ten year life, and yes they are expensive. But its a European battery design, and I had a hard time sourcing a US equal. Another nice feature is the battery has a battery mounting bracket and is is listed to use with the power supply.

What I typically would see is an AC UPS sitting loose in the bottom of the cabinet....

If you PM me with email I can send you some product brochures, rockwell has so much literature its hard to dig out what you want.


Staff member
San Francisco Bay Area, CA, USA
Electrical Engineer
The reason for interlocked doors on equipment where line voltage is present is personnel safety. There is nothing in the NEC requiring door interlocking for anything (1000V and below). You can interlock it or not, it's up to you.

NFPA 79 however DOES require it. Each disconnecting means mounted within or adjacent
to a control enclosure that contains live parts operating at
50 volts ac (rms value) or 60 volts dc or more shall be mechanically
or electrically interlocked, or both, with the control enclosure
doors so that none of the doors can be opened unless
the power is disconnected. Interlocking shall be reactivated
automatically when all the doors are closed.
Whether or not the breaker in a box with a power supply would count as an " industrial control panel" and fall under NFPA 79 or not is another matter. Probably not, and NFPA79 is not an "enforceable" standard anyway, unless a specific facility makes it so. That doesn't stop anyone from using common sense and designing for safety however. I would do it.

The most reliable type of disconnect operating mechanisms are the "flange mount" type, which stay with the ENCLOSURE when the door it opened, so misalignment of the door is not as big of an issue. That is also going to be the most expensive option and requires a specific type of enclosure. It might be overkill for this application, but you asked what is "best". Personally, I would do it with a low cost rotary thru-door operated fused disconnect with fuses sized per the power supply input protection requirements. Keep the enclosure as shallow as possible and stand off the disconnect mechanism on a bracket to keep the shaft short, that will help to keep everything aligned and working more reliably.

Side note: check your Ethernet switch instructions, because all of the industrial Ethernet switches I have seen or used require that they be powered by a Class 2 source, meaning limited to 4A (100VA if it is 24VDC). That can put a big crimp on your design if you were thinking of larger more centralized power supplies. There is a way around that by using what are called "Electronic Circuit Protectors" that go on the 24VDC side and accept a larger 24VDC source, then give you (typically) 4 separate 4A Class 2 circuits on the output. Most of the Power Supply mfrs sell those now. If you are going with Rockwell Stratix switches, they sell an Ethernet Power Supply that has an 8A output, but can be split into two separate 4A Class 2 circuits.


So just to be clear on some items.

Switch and power supply will be in the same cabinet. I'm not going to be distributing the 24vdc. That's crazy. Who would do that. I'm running 480 right into my zone cabinet and into the three phase DC power supply.

I don't need or want a UPS for this particular system. I'm not doing batteries. No batteries. Batteries are evil. Did I mention I'm not doing batteries anymore. They last ten years until one day you open a cabinet and the battery leaks all over the place at year nine. Been there once and not doing that again. If the busduct is off the equipment under it will be too. This system isn't that critical. It can stand a few minutes of reboot time.

I don't need DLR or any other ring technology either. The zones are somewhat isolated from each other. A ring won't get me anything as the fibers have to travel the same paths.

I plan to use AB network switches. Everything on the switch will be 5069 I/O. No drives or servos or safety. All discrete AC or DC I/O. Boring and slow accumulation conveyor. It's ControlLogix. Very slow RPI related to speed.

So this is essentially a slow RIO conversion. I'm replacing blue hose with Ethernet. All old 1771 I/O and miles of hose. Each blue hose gets its own zone box and its own fiber module in the Logix Chassis. This is a little bit of overkill but it also plays into how I can only rework one zone/area at a time over perhaps a 4-5 year time frame to get all the zones done. If I could do it all at once a ring might help but I cannot make that happen due to lack of funds.

I really want to go with unmanaged switches. I modeled the system in IAB using AB unmanaged switches and I'm barely raising the needle on bandwidth. I cannot rely upon our IS/IT group to manage this network. They are difficult and expensive. (The last network they built for me using the company standard cost $30k and could run a small business. Had four Cisco 24 port switches with dual fiber ring. I used ONE port on each switch to a single ENET module. "if it's touching our network it has to be Cisco!!". It cost more than the whole PLC portion of the system.) The only thing I'm afraid of here is that an unmanaged AB switch has almost zero diagnostics. I'm not a network guy so I don't know that I could solve the problem even if I had managed switches. I would consider buying one managed switch as a diagnostics tool. Crude but I could insert it into a problem zone and invite one of our IS/IT guys to investigate. Unmanaged is sort of a hammer approach. But it may work. I still have to run this by the Rockwell distributor network tech guy.

I'm going to miss RIO blue hose. There was not infrastructure. I'm just turning on ones and turning off zeros....or is it the other way around? I don't need any horsepower. Thanks Rockwell!

...and thank you regarding the 100VA limit. I might have forgotten that in my haste. Thanks for the tip on the power supplies and protector. I think I'll need that because the 3 phase input DC supplies don't go down to 100VA.

Or.....I just rely upon Radwell for another 7 years to get to my retirement. That 1771 stuff will be around for a while.


Senior Member
Why a three phase power supply? A single phase 480 power supply should be more than sufficient, along with being a heck of a lot cheaper. Just because you have all three phases there, doesn’t mean you have to use all three.


Senior Member
Meanwell WDR120 is a din rail supply that takes single phase 480 in, is 24v @ 5a, and only costs about $60. There is a little UPS module available for these which sits between the powersupply and load and connects to a pair of sla batteries.