1
\$\begingroup\$

I hope this is an easy question for you.

I am using a plastic box as a watertight enclosure for a single board computer and some accessories. I will be using an AC power source coming in through a 3 wire wet pluggable bulkhead connector. It is then converted to DC in the box to power the components (like the SBC, a serial-to-usb converter, etc). The AC/DC converter (Mean well brand) has a place for attaching a ground.

My question is: will using the ground screw on the AC/DC converter be enough to ensure this box is safely grounded? My gut says yes, especially since the box is plastic, but I am going to be using this in a saltwater environment. I really don't want to kill myself so I'd like to be extra sure before I deploy this baby.

Thanks much!

EDIT: Unless the ship goes down, this box will not be submerged. It will only be exposed to saltwater spray during cleanup.

\$\endgroup\$
  • 1
    \$\begingroup\$ No, AC power going into a plastic box in a saltwater environment is not necessarily safe. For example, a single point of failure in the seal, which can degrade over time can result in a hazardous condition. Just grounding the ground screw on the AC/DC supply is not going to prevent a hazard. \$\endgroup\$ – John D Feb 6 '18 at 23:13
  • \$\begingroup\$ and how will you remove the heat thru plastic ? \$\endgroup\$ – Sunnyskyguy EE75 Feb 6 '18 at 23:38
  • \$\begingroup\$ Thanks John D. Can you clarify what you mean by 'hazard'? I know there is the potential for water intrusion, broken wires, sentient computer boards, etc. Par for the course when dealing with electronics on a ship. I guess a better question is: will the box be safe to touch until it can be removed from AC power if one of those potentials happens to occur? Is using the grounding screw and a plastic case enough? If not, what else should I do? - Thanks \$\endgroup\$ – imno1 Feb 6 '18 at 23:44
  • 1
    \$\begingroup\$ It's not uncommon for equipment enclosures made of glass reinforced plastic (GRP) to be used in exposed areas of a ship or offshore vessel. IP56 is usually the minimum IP level allowed in wet areas. Inside the enclosure, you can use a brass plate as a backup surface for metal bulkhead connectors; it provides a PE connection point inside the enclosure. Any exposed metal surfaces must be bonded to the common PE point, which must have a path back to an earthing point on the vessel. Again, check your ship's classification for the standards you need to adhere to! \$\endgroup\$ – AlmostDone Feb 7 '18 at 0:15
  • 1
    \$\begingroup\$ Hazard means the potential for injury. In this case saltwater is a fairly good conductor so a leak that allows a salt water bridge between a live conductor and the outside of the enclosure could cause electric shock. @AlmostDone is correct, you have to follow the applicable standards, and after following the standards I would add a GFI for an added layer of safety. \$\endgroup\$ – John D Feb 7 '18 at 0:38
0
\$\begingroup\$

If the screw is exposed out of the plastic box, it is an accessible conductor. If it is screwed with the Mean well AC-DC converter, the easy way I think is to make sure the converter meet safety standard. For example to check whether it has EN61204 safety standard certificate. If so then you are sure the converter grounding is okay. Next is you have to make sure the screw contact well with the grounding. However, if you can loose the screw without opening the plastis box, you cannot just ground the screw. I would keep the screw inside the plastic box and do not let any metal exposed out of the box.

However, to make the box safe, grounding or not is not the only factor to consider. As you said you are using a plastic box, if there is not accessible conductor (aka exposed metel), you do not need grounding, just like the hair dryer in bathroom. Another important safety factor is the creepage and clearance between you electronics and the accessible surface (the plastic box surface). It depends on the working voltage level, rated impulse voltage level etc. For example, with reference to EN60335, the clearance for a common 230V appliance is 3mm. However as your product will be exposed in a salty environment, I will use the next higher level which is 6mm. The thickness of the plastic better be more than 1.2mm. And even better is you have double insulation, that means you have two plastic box with total clearance of 6mm.

\$\endgroup\$
  • \$\begingroup\$ Thank you, eepty. This is very useful information. The screw is inside of the box, on the converter itself (looking into the EN standard now). As for clearance between the electronics and the box, it's on the order of inches, not mm. There will be plenty of room on the top and all sides. As for the bottom, I am planning on have a plastic board setting off of the bottom of the box a little over an inch to mount everything to. The box itself if about 7-8mm thick. \$\endgroup\$ – imno1 Feb 7 '18 at 18:05
0
\$\begingroup\$

I am going to be using this in a saltwater environment. I really don't want to kill myself so I'd like to be extra sure before I deploy this baby.

I have a large Koi pond with underwater pumps driven by regular AC and I don't want dead fish but, luckily, legislation in the UK means that I must fit a residual current device (GCFI or ground fault current interrupter in the US) to outgoing AC connections for my pond.

This means that if water gets where it isn't intended (such as the live wire) and a potentially excessive ground current is caused, the interrupter trips the whole AC supply rendering it safe.

\$\endgroup\$
  • \$\begingroup\$ Thanks Andy aka. I am 92% sure the entire ship operates with GCFIs, but I will take that question to them when I go next week. \$\endgroup\$ – imno1 Feb 7 '18 at 18:07

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.