metal bathtub grounding

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sparkproof

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I'm working on the renovation of an existing apartment building that has metal tubs. The inspector noted that a metal box is mounted to metal studs, and the studs are contacting the metal bathtub. The contractor has proposed to install an insulating material between the studs and the tub in order to electrically isolate the tub from any potential voltage from the circuit in the box. Now they want a letter from the engineer (me) approving this approach. I've requested a code citation from the inspector, and he can't provide one. I'm concerned about weighing in on an approach I'm not sure makes anything safer. Could it potentially make it worse, if the circuit in the box is now at a different potential than the tub? Interested to hear everyone's thoughts.
 
My (not engineered) gut response is that if there is a risk of a circuit energizing the metal tub, then the proper response is to bond the metal tub to all nearby grounded metal, not try to insulate things

Jon
 
Only criteria and solution found is in 250.4(A)(4). Bonding of electrically conductive materials that are likely to become energized.
So question is, Is the tub "likely to become energized"?
If yes, the only solution within code is to bond. Like Jon said.
 
If the metal tub is in contact with a grounded metal stud than how can it possibly become energized by the stud that's already grounded by its connection to a grounded metal box?
 
infinity said:
If the metal tub is in contact with a grounded metal stud than how can it possibly become energized by the stud that's already grounded by its connection to a grounded metal box?
Click to expand...
Exactly. It's bonded by the EGC of the circuit "likely" to energize it.
 
LarryFine said:
Exactly. It's bonded by the EGC of the circuit "likely" to energize it.
Click to expand...
The thinking on this one by the inspector is backwards. His thought: "The inspector noted that a metal box is mounted to metal studs, and the studs are contacting the metal bathtub".

If the metal studs are not grounded and likely to become energized then the tub could possible become energized. The other way around (grounded studs in contact with the tub) isn't an issue.
 
I would not want to be sitting in a metal tub that is attached to a copper wire going directly into a 240 volt electric panel. Just something about that gives me the heebie-jeebies.

I like Infinity's theory better.
 
winnie said:
My (not engineered) gut response is that if there is a risk of a circuit energizing the metal tub, then the proper response is to bond the metal tub to all nearby grounded metal, not try to insulate things
Click to expand...
I would think the grounded water system if metallic should be sufficient, Like others mentioned I don't think isolation would be better than direct bonding, theoretically the path of stray or fault current should seek out less resistive paths, not linger in Grampa's bath water.
 
infinity said:
If the metal tub is in contact with a grounded metal stud than how can it possibly become energized by the stud that's already grounded by its connection to a grounded metal box?
Click to expand...
If the source of potentially energizing the tub was properly bonded originally then at best the chance of energizing the tub is 3rd or 4th removed from the source and very little likelyhood of energizing the tub.
My thought, or question; Why is the tub attached or contacting the metal studs? There is no durarock or sheetrock separating the tub from the metal studs?
If the potential is truly that high, the safety aspect of it being a bathing tub in question, to me would mean not just bonding, as that would simply serve to increase the likelyhood of incidentally energizing of the tub, but to include GFCI protection on the circuit of potential energy. (Not in anyway saying it has to be done, but that bonding is not the cure all that the inspector is implying to making the tub safe.)
 
I'll give you a very common scenario of it being energized, be it a low trickle.

If that tub was on a slab in the basement with metal drains going deep into the ground. Say your (ground line) phone is bonded to the service neutral closer to the tub then the utilities.
If you're taking a shower and stand on the drain while touching the water valve. If someone is on the phone you might feel a trickle. 24volts or less.
If the utilities neutral was lost, you may feel a lot more.

I use the above scenario to explain to non-electricians why you need 4 wires to a sub panel. ;)
 
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