# Working in enclosures with 120V live and 24V control

#### MicrobicTomb

At a new company I have been introduced to a much stricter policy regarding electrical work of any kind. Basically they do not want anyone working on anything 50V or higher while it is energized or we bring in an electrician. My experience has typically been high voltage (220 and up) in one enclosure with mains disconnect or other protection, which then feeds into another enclosure stepped down with a transformer or two to 120VAC and/or 24VDC for control.

Currently designing a control cabinet to house a 120V -> 24VDC power supply which will have a circuit breaker on the input feeding the power supply. The rest of the items will be typical, PLC, I/O, terminal blocks, etc. The question was raised as to whether or not the restriction is too much and I wanted to get some other opinions on the matter. My concern is from a design/build/programming standpoint and then feasibility of troubleshooting in the future.

Other options I could do to circumvent this issue entirely:
Adding in external communications ports so the cabinet would not need to be opened
Adding a second enclosure to house the power supply/circuit breaker and connect the 24V to this cabinet

Both of which add cost that I'm on the fence of is it really necessary. I understand the potential dangers of 120VAC but from my experience if the installation is done properly, it would not be an issue for a qualified person to open the cabinet under power and perform basic testing/programming.

Thoughts?

#### Carl

Sounds like health and safety gone mad.

#### CSA

Danger Rangers to the rescue!

You have to feel a little sorry for them. They continually feel like they have to justify their salary. That's a lot of pressure to always be under.

I long for the days of apprenticeships. Instead, we have Danger Rangers.

#### patrickduis

Sounds like a health & safety officer that wants rotating doors on all intersections of roads so no collapses can occur and the health & safety officer is God above anything else (or became God after an accident in the past).
I worked in many 400Vac 3 phase installations and yeah (like many of of) I got my share of touching live 400V......even 690V once...that hurt bad.
I had to do many repairs on installations under voltage but you need to look out very carefully and measure everything with a proper multimeter before touching it.
You could also consider buying VDE style tools, gloves and protection that have 1000V insulation.

Another option is putting the <50V control items in a separate compartment that may be opened and the >50V stuff in another one that remains closed when live.

Engineering is about problem solving, not about putting everything to a standstill so nobody gets hurt.
Then we must also start driving max. 30km/h with all cars and motorbikes, to prevent accidents.

#### BillyMac

A perspective from an Ontario Canada electrician and a JHSC member... as well-meaning as locking out the 120 to look at the 24 is, it's a pain in the... and virtually impossible without re-design of panels or flexibility in the safety rule application. I have gained some acceptance from H&S supervisors by adding clear Lexan barriers over the AC wiring. I also work as safe as I can: sleeves rolled down and leather gloves.
I can understand a facility's firm stance on the zero energy/LOTO rules. If I get zapped (or worse) , someone on the management team will be held accountable. It may not seem fair or right, but that's the law.

#### patrickduis

Yes I also used lexan plates over the 230V/400V wiring. That is a good solution.

#### MicrobicTomb

I understand both sides. Trying to come up with solutions to allow easier troubleshooting/programming with out re-designing enclosures is really all I can do, safety is in charge. The lexan plates are a good idea. I'm looking into one currently to just guard the terminals. To me, I think it's a bit much. If you're going to get shocked on a 120V circuit that has finger safe terminals and no other exposed energized parts then you're asking for it at that point.

#### CSA

Gotta do what you gotta do.

#### Carl

When it comes to health and safety vs tech person...tech person loses
health and safety vs bean counter....bean counter wins

Ask for training courses ...those I week courses that cost \$ there might be some better movement for you if you want to develop yourself as a controls guy you need to be able to do certain things...must be very frustrating.

#### patrickduis

The nice thing about Rittal panels is that you can order them with the perforated square holes on the sides. In these whole you can put the bolts that center themselves automatically on the screws, they are inside a square housing (don't know the exact word for it). And on these rails you can, very nicely, put a few lexan plates. Make them in separate parts, otherwise the plates get too big. Also put one just over for example the fuses, because that's where you are often working.
When there is something wrong you just have to remove the lexan plate over the fuses.

Or use CE certified switching gear that is already touch-safe, and Rittal PLS rail systems (that can have shielding on them, original from Rittal) for power distribution.

Can you post a picture of the inside of a panel you need to shield, that enables us better to give you good advice.

Here is a picture of a thyristor panel I made many times before for controlling power to 100A SiC rods inside a Brands bottom hearth kiln that was used for producing ferrite ceramics (filter material/power supply transformers).
To the left is the main switch and under that a big relay.
Middle and left are the thyristors.
In the bottom is a Rittal PLS bus-bar that is shielded.

In front of many parts are the Lexan plats. Now look out with those, if you just cut them, the sides are really RAZOR SHARP.
So what we always did, is bending the top and bottom so they are no sharp anymore.
This also gives them more strength against bending.

I built about a few 100s of these cabinets, large and small.

The very experienced guy to the right was my chief electrical engineer. A really nice guy that had a calender the last 2 years before his retirement. Every day he marked a day on that calendar.
When he finally retired after 50 years of working he was so happy. But just a few months after his retirement he got cancer and died in just a few months time unfortunately.

I still remember finding ashes of his cigars in control cabinets that he built............for me that was a quality mark: if I found ashes there I knew Cees made the cabinet and all was built properly. I didn't have to look for stupid mistakes.
Still miss him a lot.

#### MicrobicTomb

NFPA 70E training is the qualification we need on site but it doesn't change the ability to work on live >50V. It gets into the arc flash boundary limits and gear. I've been doing industrial controls for 5 years now and have been in electronics/communications for 20 years. This is the first company I've worked for that took this that seriously.

Sorry to hear that Patrick. I've known a few "Cees" in my day as well, always a pleasure to see their signature work.

#### Carl

Is that some ATEX enviroment??

#### rplusich

At a new company I have been introduced to a much stricter policy regarding electrical work of any kind. Basically they do not want anyone working on anything 50V or higher while it is energized or we bring in an electrician. My experience has typically been high voltage (220 and up) in one enclosure with mains disconnect or other protection, which then feeds into another enclosure stepped down with a transformer or two to 120VAC and/or 24VDC for control.

Currently designing a control cabinet to house a 120V -> 24VDC power supply which will have a circuit breaker on the input feeding the power supply. The rest of the items will be typical, PLC, I/O, terminal blocks, etc. The question was raised as to whether or not the restriction is too much and I wanted to get some other opinions on the matter. My concern is from a design/build/programming standpoint and then feasibility of troubleshooting in the future.

Other options I could do to circumvent this issue entirely:
Adding in external communications ports so the cabinet would not need to be opened
Adding a second enclosure to house the power supply/circuit breaker and connect the 24V to this cabinet

Both of which add cost that I'm on the fence of is it really necessary. I understand the potential dangers of 120VAC but from my experience if the installation is done properly, it would not be an issue for a qualified person to open the cabinet under power and perform basic testing/programming.

Thoughts?
the external comm port is something I include in all the panels we build (I like the ones that have a power receptacle as well, to keep the laptop alive just in case the battery is a little flat, or you’re there for a while). I’d rather build in a permanent connection to the controller and have a cheap inexpensive sacrificial port to jack into for maintenance and programming
and I really like running all my control 24vdc, but more for convenience than safety. we usually have 24vdc present for sensors and transmitters, get better density out of 24vdc IO, & so we run our pilot devices, relays, and contactors with 24vdc coils, and let everything go 24.
for those who suggest lexan barriers: make sure they’re listed for the application, or you’re in a whole new can of worms (but I do like the idea).

#### BillyMac

I've sold the idea of a lexan barrier as a prevention against accidental contact/shock. It's non-conductive property and distance from exposed terminals - usually 4"-6" standoffs - is critical. It is not intended to address the potential of arc flash/blast effects.