Is 40V Induced Current from Heat Mat Causing Shocks in Stainless Steel Shower Drain?

Hello all, mood here, 
Ceramic tile and stone contractor by trade, Jack hack of most anything else.
I recently install a very nice walk in shower with beautiful 12x24 slate look porcelain and marble tiles. Rain head shower head in ceiling, linear style drain system completely isolated from what comes next.
Outside of this again beautiful slate look tile floor over heat mat unit. 
Heat mat goes in without a hitch one of the nicest units I've seen, (I was not responsible for the electrical connections). Licensed electric co.
Anyway got a call a couple weeks back to tell me every time client took a shower they were getting shocked. 
Tech people from mat Co. H.B.Fuller said there was an induced flow of electric from the mat of 40 volts charging the stainless steel drain. They are telling us the shower floor has to be demo'd and a UL ground clamp has to be applied to the drain system.
If anyone pipes in, I can gladly elaborate more.
Is this possible?
Thank you in advance
Neil in Cincinnati Ohio

Neil
I've put in a couple of underfloor heating systems and I would NEVER put one in a shower.  But I presume your licensed electrician made sure this was OK before he installed it?  Does the manufacturer say that the earthing should have been done in the installation instructions?  A lot of questions there.  I'd say 40 volts (I presume from 120V supply) is a lot to be (induced) and would rather suspect leakage.  without knowing more about the heat mat I can't say, though.I would get your electrician back and ask them nicely if they can help you do some tests.  If the cold water pipes are metal and grounded you may be able to test by running a clip lead from the cold tap to the drain and see if that stops it And if the instructions say that any metal should be earthed, and your electrician didn't, I'd point out that he is liable for the costs of correcting this.  Where electricity and water are concerned, you don't mess around, and your electrician could be liable for some serious court stuff if someone was injured or killed by this, so you have a bit of leverage there I'd say.If you have a link to the mat specs I'll take a look there.

Floor plan, large black rectangles are heat mats.
Small rectangle is drain inside shower, drain system is PVC isolated in it's entirety by a liquid rubber shower pan  liner.

This is a scary situation.  It seems to me that the electrician should be taking the lead on resolving the problem. You don't want to mess around with something that could possibly electrocute someone.  I'm not even going to suggest anything you might try to resolve the problem because I'm concerned about possible liability.Please make sure the homeowner turns the thing off until this is resolved!-Rick

Here is a link to my tile forum thread if anyone wants to chime in.https://www.johnbridge.com/vbulletin/showthread.php?p=1502472&posted=1#post1502472
TIA

Let me get this right, the actual shower is the area outlined in red below?  The rest of the area is a larger room presumably with vanities (wash hand basins), towel rails, etc?In that case I am not as horrified as at first, but I still think you have a big problem somewhere and would agree with Rick below that (a) your electrician should be resolving this and (b) switch it off until fixed.  Again, you don't mess around with electricity and water!

Neil, just had a look at your Tile Forum thread and a lot of good questions are raised there.  It is possible that the shock voltage is coming from the water pipes / shower and not the drain, the drain is acting as a partial earth when wet.Your electrician needs to get an AC voltmeter with some extended leads, connect one to a known good ground and probe around the shower (drain grate, taps, shower head, anything else metal) to find out what actually has this voltage on it.  The fact that the customer is feeling a shock when touching the drain grate does not necessarily mean that that is where the problem is.Again, heed Rick's very good advice: "It seems to me that the electrician should be taking the lead on resolving the problem. You don't want to mess around with something that could possibly electrocute someone."   It may not be the electrician's fault directly, but he's far better equipped than you, or the customer, or even the manufacturer, to resolve this problem.

Thanks for the serious concern fellas, I'm just trying to shed light on potential area's to address before I'm asked to demo this floor.  I'm just the tile contractor who has never seen this problem before and I've installed at least 25 of these heat mats of various types and wired to completion.The questions you ask are being presented to the Homeowner for their thoughts.

The Home Owner sent this to me after I had directed him to look over this thread.
Thanks Guys a lot.
Here
are some bullet points for the discussion for questions that were being asked.

 

 

Heating mat is 100% outside the shower areaLicensed Electrician has been doing the testing on site
with guidance/input from the mat manufacturer (also an electrical
engineer)Heating system is on a dedicated GFCI, and that GFCI
circuit employs a manufacturer recommended thermostat with more sensitive
current leakage sensitivity.There is a +/-40V current present only when the heating
mat is actually heating (calling for heat).  Circuit can be on with
no shock, only when the mat is actually being supplied with power from the
thermostat is the voltage presentIf you disconnect one of the mats you see a drop in the
current, not exactly by half, but something in that neighborhoodConnection of the drain to any ground (when mat is
heating) will show the current.  For example, if you connect the
drain to the house electrical ground, or if you are standing in the shower
with some water and you touch the metal faucet (plumbing supply is
grounded).  Alternately, if you are in the shower with no water
present (thus not conductive to the drain) you will not get the
shock.  The only place you get the voltage is by way of connection of
the drain assembly to ground.No voltage is present if you connect the shower valve
to any other metal grounded/not grounded item in the bathYou can imagine, we have run leads from everything to
everything to identify this.  Only common denominator is the shower drain
which has zero connection to anything else conductive.

Neil, thanks for the feedback, it's always good when we get responses to our replies.OK, from what I gathered from the above, you only get shocks of the drain, only when the heater is actually heating, and it goes away if you ground the drain?If that's the case. next question is, how much current flows if you ground the drain?  You have told us that there's 40V from the drain to ground.  If you connect the drain to ground, how much current flows?  I would imagine not much (a few milliamps maybe, in view of the fact the the GFCI doesn't trip)?)  Your electrician should be able to measure this.If it is only a few mA, I'd suggest that you think about putting in a permanent ground to the drain.  You may be able to do this by scraping out some grouting and putting the wire in there, or you may need to lift some tiles (which would be a major PITA I know...).The only other suggestion I would make is to use a non-metallic drain, if such a thing is available?  But I'm still unhappy about this, if one of the mats is faulty there's a potential for it to get worse over time, and cause major problems.Cheers / David