What would be the purpose of the ground being on the outside of the device
Depends on many factors.
I have never seen it before. trying to figure out if the current install is correct.
I would not be able to ascertain from a picture, whether or not that installation is correct. For all I know, additional bonding may be required, hence the reason for the external ground screw - but I'm just speculating.
Check the Manual of the Motor.
It will probably mention something to do with UL compliance External Bonding Connection Point for Conduit Bushings.
Following Article 250, there are specific conditions where the Conduit can qualify as the EGC.
Is it even electrical conduit?
My judgement, based on visuals;
* This is a Mod Motor (24V AC), a Modulating Motor controlling a Damper (technically not under NEC).
* The clear hoses behind are tigon tubing for a Magnehelic comparing pressure before/after the Damper.
* It's relatively clean other than rust where the Water Pipe was threaded because it's outside.
* The Water Pipe, which can be used underground, qualifies to the installer as 'outside' electrical conduit.
* They forgot the Teflon Tape to keep the moisture from getting in due to temperature changes.
* At least there is not evidence of using Pipe Wrench to tighten - ultimately cracking the Motor's Cast Housing.
I don't know whether to laugh, or cry.
That is obviously an MSA explosion-proof equipment housing.
It is most likely a gas detector. The Tygon tubing is likely used to convey sample gas to/from the sensor head, to convey calibration gas to the sensor during calibration, or something similar. (Tygon is often used since it does not absorb the gases being conveyed; this would affect the reading of the detector.)
The external grounding point is a requirement of UL 1203 and other similar standards for explosion-proof equipment.
Once it may have been used as an actual grounding connection. However, 1203 specifically says that the terminal is NOT supposed to be marked as a grounding point, though it can be so marked so long as the instructions clearly state that it is not to be used for such a purpose. (Also, it is very easy to misread this requirement to say the terminal IS supposed to be marked as ground.)
Regardless, the terminal is still required, as it is occasionally used to bond the housing to adjacent grounded metal to prevent electrostatic discharge, or prevent a case where adjacent parts might be at significantly different potentials. Such arcing, while not directly hazardous, can easily cause ignition of an explosive atmosphere that (presumably) surrounds the enclosure. 1203 calls it a "supplemental equipotential bonding connection".
The connection is not normally needed since the enclosure itself is usually in solid contact with grounded metal/metallic parts, plus the metallic conduit connection. These typically form a perfectly good path for discharging ESD and preventing large potential differences between adjacent metal parts. But there can be rare cases where adjacent metal might be temporarily energized to a high potential relative to the MSA box; this could cause an arc that can also cause an explosion.
1203 also requires that there be an internal grounding connection, as in all other equipment. So the external terminal shown is definitely not the intended grounding point.
From this, the external connection point has to be there, but is most likely not needed for the particular install. The device is most likely wired with a "proper" internal grounding conductor.
The conduit forms part of the explosion-proof protection, and also must have good electrical contact to prevent sparking (explosion) if fault current runs through the conduit system. Therefore - and typically - the threads are not allowed to have Teflon or any other similar material applied. Waterproofing such systems can be an issue.
Per Art 501.10(E) (or thereabouts), the conduit must be installed wrench-tight. Otherwise, the explosion-proof protection at the threads is compromised.
It would seem the unused threads are rusty, which is not unusual. External rust won't affect the explosion-proof protection. Rust on the internal threads probably won't either, but excessive corrosion might cause pitting or gaps in the threads, which might cause issues.
If memory serves, conduit is not specifically rated as explosion-proof, but does (usually) have to be RMC or IMC type. I cannot tell if the black pipe used is actual RMC or IMC, or if it is something else.
Other than that, I don't see anything in the photo to indicate this installation was done incorrectly, or that the equipment is not being used correctly. But the perspective is obviously limited.
I have a crazy idea. Why don't we ask the OP for more detail, rather than to guess what it is and whether it's installed correctly, by nothing more than a single picture (thanks for the picture, by the way)?
Quite often the installer uses the conduit body as the EGC. If the installer runs a flexible conduit to the motor (or any other component) a EGC can be run on the outside of the flexible conduit (up to 6'), I haven't seen a bonding point on the outside of the equipment before. Generally I've seen sealtight connectors with an integral lug.
250.102 Grounded Conductor, Bonding Conductors,and Jumpers.
(E) Installation. Bonding jumpers or conductors and equipmentbonding jumpers shall be permitted to be installed insideor outside of a raceway or an enclosure.
(2) Outside a Raceway or an Enclosure. If installed onthe outside, the length of the bonding jumper or conductor or
equipment bonding jumper shall not exceed 1.8m (6') and
shall be routed with the raceway or enclosure.
Some people are always thinking outside the box.
for this specific application, it is a bonding screw.
Is that rust on the threads...or anti-seize compound?
in my opinion it looks like pvc coated ridged metal conduit ( commonly called rob roy or Ocal ) and copper coat used on the threads for conductivity and lubrication.
Is that a seam in the pipe, or reflection from the lights?
pvc coated pipe usually has a seam, it is from the manufacturing process it's more of just a line not a seam. the rigid pipe under it does not.
That doesn’t look like Rust to me, it looks like a copper based anti-corrosive coating , it’s also on the lower plug.
I stand corrected on the "rust".
However, it's still not a motor. The nameplate clearly says "MSA A-Ultimax", which is a gas detector:
A-ULTIMAX-GP-G-00-0-2-S0-0-0-0-0-1-0-0 by MSA - Buy or Repair at Radwell - Radwell.com
Ah yes, I concur - not rust. And it's a Gas Monitor, not a Mod Motor.
So what's the purpose of the Ground being on the outside of the Device? - Check the Manual:
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