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I remember seeing a post by you with regard to equiopotential bonding that really makes me wonder "WHY" my niche is a classic example it is PVC, low voltage light, but yet when I "Bond" it to the Equiopotential Grid, I will have also tied it into my grounding system as the lug on the outside of the niche for bonding is itself bonded to the "ground" lug on the inside of the niche. So if something goes wrong and goes to ground, the whole Equipotential grid and everything connected to it could go live right? doesn't make sense. I know its code, and I am doing it, but I wish the niche wasn't bonded to the grounding lug.
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First you must follow the manufacturers instructions for installing the forming shell . Second there are some PVC wet niche forming shells that do not require the #8 bond wire or bonding to the grid. These are generally sold as matching pair with the LOW VOLTAGE light. Which is also listed as non grounding and has a locking PVC face plate exposed to the pool water. Also PVC conduit from the jb to the forming shell. And also a pvc junction box not a metal one. Other wise the bonding lug on the pvc forming shell is used to bond the metal junction box to the grid. See diagram in next long winded post after this one.
Your forming shell and luminaire should look something like the one at the end of my long dissertation......so grab a beer....
Start thinking bonding lug (a place to connect to in order to bond or join metal together as one) ) and not ground lug (a place to connect metal to a facilitate a fault current path). Not a 100% perfect comparison but I think looking at it this way will make things more understandable since you have a PVC forming shell.
Also at the end of this is a excerpt from NFPA 70 and the UL 676 governing pools documenting the part about some situations not requiring the #8 bonding wire so you don't think I pulled that one out of my ***. Take note it says "low voltage system" meaning junction box is pvc, forming shell is pvc and light is a non grounding type.The only change to that language is they now are calling it a bonding wire instead of an equipment grounding conductor. Calling it the later was misrepresenting the purpose of the #8 which is to bond the forming shell and the junction box to the grid...especially if a line voltage light and PVC conduit to a
metal forming shell. So the primary purpose is bonding with other possible duties as described below. The connection of the #8 to the
equipment ground coming from the wet niche light cord and transformer at the junction box is only incindental and not intentional. A result of bonding the metal of the junction box to the equipotential grid via the #8.
So assuming that your forming shell is pvc and has a bonding lug and is not a listed combination with a non-grounding wet niche light I'll try to explain as it is common for people to have a difficult time with this concept. I'm going to para phrase from some of my literature to make it faster for me to explain this bonding in the forming shell. First thing to understand is that any faults in the supply conductors to themselves or metal out to the metal junction box will cause the gfci breaker to trip out since you have one on the low voltage light circuit. Even though gfci isn't required for this low voltage light of yours it is an excellent idea. However from the JB to the forming shell its a different story. So see if the below helps you understand if not I'll give it another try.
The equipment grounding conductor in the flexible cord of the wet-niche luminaire may be relatively small (as little as 16 AWG in accordance with the
NEC), and may have a length of one hundred feet or more. The relatively small AWG and long length will result in increased impedance through the flexible cord equipment grounding conductor.
If the electrical bond between the luminaire and the metal forming shell is poor or absent, in an electrical fault condition within the luminaire this increased flexible cord impedance causes a relatively greater voltage potential between the dead metal of the luminaire and other dead metal connected to the premises’ equipment grounding conductor.
The supplemental equipment grounding conductor in the form of corrosion resistant metallic conduit connected to the forming shell or the 8 AWG copper conductor routed with the flexible cord in nonmetallic conduit provides a second, more conductive, path for fault current originating in the luminaire. If the luminaire securement does not provide an effective electrical bond to the forming shell, there is an equally ineffective electrical bond to the supplemental equipment grounding conductor.
Section 680.26 of the
NEC requires essentially all of the dead metal within 1.5 m (5 ft) horizontally from the pool wall and the dead metal of equipment associated with the pool water circulating system to be connected to a low-impedance common bonding grid, using a solid copper conductor not smaller than 8 AWG. Likewise, the electrical impedance of the electrical bond from the dead metal parts of the wet-niche luminaire to the pool bonding grid must be low. UL 676 requires the impedance between dead metal parts of a luminaire and the 8 AWG pool bonding grid conductor connected to the applicable terminal on the forming shell not to be more than 0.020 ohm. For perspective, this is approximately the DC resistance of 10 m of solid 8 AWG uncoated copper conductor at 25C, a relatively low value. So in the case of a metal forming shell the #8 plays double duty as a redundant equipment ground for fault current in the event of a fault with the lunimaire that cannot be sufficiently handled by it's equipment ground wire in the cord for what ever reason. So look at it as the metal of the forming shell is providing an alternate path for a fault at the light to get to the bonding lug and utilize the #8 for a alternate fault path fault back up to the equipment grounds in the junction box so a breaker will trip. All this is primarily directed at line voltage lights (120v) and metal forming shells.
With a PVC forming shell and a low voltage light like you have this all is rather irrelevant. In which case the lug provided on the forming shell is just a means to bond the metal of the junction box (deck box) to the equipotential grid via the #8 bond wire. You may also have a metal trim ring for the lamp which will bond to that lug with a metal strap....see the diagram.
Lets take a break...Cheers..
The equipotential grid is primarily concerned with voltage gradients that occur from external means other than the pool electrical equipment and supply conductors. By this I mean underground utilities or service laterals that are breached or failing causing current to leak into the soil in the vicinty of the pool. These voltage gradients cause touch potential of metal parts to become elevated to the point you may get electrocuted if you come in contact with a piece of metal at a different potential than the other metal around the pool as the result of a voltage gradient in or around the pool. The bonding of all the metal brings all metal to the
SAME potential and then by defintion current cannot flow if touched by you as there is no difference of potential on that metal in respect to the other metal. Whew....the end.
so check your mail box around thursday/friday. 
Linear Mode
