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The Danger of Voltage Gradients in the Aquatics Environment

Posted By Bob Herzig, Friday, November 01, 2002
Updated: Thursday, February 14, 2013

Hidden beneath the glamour and excitement of an aquatics complex is a detailed and essential safety system. This frequently misunderstood network is the swimming pool bonding grid. Article 680 of NEC 2002 covers this system and was revised in this latest edition. Several sections changed paragraph numbers and other information was reformatted to provide clearer interpretation of this section. Section 680.26, Bonding, was formerly 680.22 in NEC 1999. The intent of this section still remains to provide an equipotential plane in the aquatics environment for safety.

Why the Concern or Need for 680.26 on Bonding?

The answer to this question can best be explained with a real life case scenario. A lifeguard in the Kansas City area was recently frozen to her stand and unable to move or call out for help when a voltage gradient was introduced into the aquatics complex she was guarding. Here’s what happened.

The pool was in operation in early June. It was hot and humid. A construction company was in the process of digging trenches immediately adjacent to the pool when a trackhoe operator ran his boom into an overhead utility power line. The voltage of the system was 12,470 volts phase-to-phase and 7,200 volts to ground. This large piece of metal equipment conducted the voltage of the utility line down to the earth adjacent to the pool, energizing the ground with a substantial voltage gradient. This gradient began seeking out the lowest ground plane to be dissipated. Think of water running down hill. Electricity, like water, seeks the lowest point in the system. The large community pool, filled with conductive ionic water and constructed of large quantities of reinforcement steel, provided a major ground plane. The gradient from the utility line, through the trackhoe and into the earth began migrating to the pool. The lifeguard stand was located in a direct path between the gradient and the pool ground plane. Immediately, the voltage gradient froze the lifeguard when a touch potential developed between the two stainless steel (conductive) arms of her lifeguard stand. The current developed exceeded the let-go threshold through her body, and she was unable to move out of this dangerous situation. Simultaneously, the trackhoe operator noticed that his co-worker, who was in the pit below the trackhoe, was also appeared to be experiencing electrical shock. He was shaking as his hands were touching the earth on the side of the pit. Upon seeing his co-worker in this condition, the trackhoe operator lowered his boom. Pool patrons heard an arc when the boom broke contact with the energized utility line, freeing both the lifeguard and the co-worker.

Why the Lifeguard Was Shocked

An ensuing test showed that one metal arm of the lifeguard’s stand was tied (or bonded) to the pool bonding grid. The other arm was floating or isolated (not on the grid) but in contact with the earth under the lifeguard stand. This is a violation of the bonding requirements of NEC 680.26. This floating or isolated arm began picking up the voltage gradient introduced by the adjoining utility line and trackhoe. The other bonded arm of the stand provided a ground return that allowed current to flow through her body. (Note: Lifeguards are constantly applying sunscreen and are often wet with perspiration while performing their jobs. This environment adds to a lower resistive contact with the metal surfaces they frequently touch around the pool.)

What Would Have Prevented this Occurrence

Had the lifeguard stand been bonded together (connected with a low impedance) per 680.26, both arms on the stand would be maintained at the same voltage potential. The gradient would have still entered the pool, but as the stand began to elevate in potential it would have done so equally on both sides. As a result, the lifeguard would have been protected and probably unaware of any danger. An analogy is a squirrel running along an overhead power line. The front feet and back feet of the squirrel are at 12,470 volts each. Since both sets of feet are at the same potential and there is no return ground path, there is no net flow of current. The squirrel is safe.

How Does the Electrical Community Apply This Article to Assure a Safe System?

Electrical inspectors, contractors, and engineers are faced with how to interpret this section. The engineer must design a proper bonding grid that meets the minimum requirements of the Code. The electrical contractor/installer must set up this grid using proper bonding materials and methods. And the electrical inspector has the tough job of inspecting this grid to ensure that it has been designed and installed according to the NEC. Since this system is passive in nature, only operating when a voltage gradient is present, it is often designed, installed and inspected improperly. After conducting over twenty bonding surveys for new and existing aquatic complexes, I have come to several conclusions. Let’s first start by reviewing the minimum requirements in the NEC.

A Closer Look at the NEC?

Section 680.26(B) outlines the items that are required to be bonded together. These include: (1) metallic structural components, (2) underwater lighting, (3) metal fittings, (4) electrical equipment, and (5) metal wiring methods and equipment. My experience shows that section (5) is often the problem area. This section requires the following metal components to be bonded to the bonding grid: metal sheathed cables, raceways, pipes, and all other metal parts within the following distances of the pool:

(1) Within 1.5 m (5 ft) horizontally of the inside pool wall
(2) Within 3.7 m (12 ft) vertically above the pool or observation stands, towers, platforms, or any diving structure

Many of these items are either missed during the original construction or compromised by future work done around the pool.

Experience Can Teach Us How to Inspect

Three major situations occur that cause violations in this section. These include (1) not bonding all parts or sections of a piece of equipment that is within five feet of the inside pool wall during original construction, (2) adding or moving items to the pool after the original construction and/or (3) cutting or repairing the pool deck.

Bond All Parts of the Equipment

The first situation that compromises safety is not bonding all parts of a pool piece of equipment during the original construction. An entry/exit ladder within a pool normally requires two anchor sockets embedded within the concrete deck to anchor the ladder. Many ladders today have stainless handrails with plastic steps. The two handrails are electrically isolated because of the plastic steps. Many contractors will only bond one anchor socket for each ladder. An inspector sees the bonding conductor present at the handrail and connected to one handrail and assumes the whole assembly is bonded. The result may leave one handrail bonded to the grid and another floating to pick a voltage gradient. This is a touch potential shock situation waiting to happen. The solution (and the rule) is to require every anchor point to be bonded around the pool deck. This applies to handrails, ladders, lifeguard stands, canopies, and so forth. (Note: items that are bonded with lugs not labeled and listed for the chlorinated environment will often deteriorate and cause a failure in the bond connection [see 680.26(C)]. These connections should be suitable for the purpose.

Adding Amenities After the Original Construction

The second situation is when items are added or moved. It is common for a pool to have lifeguard stands installed when the pool is built. At a later date, when more money becomes available, many pools will have accessory items such as lifeguard canopies (sunshade covers for the guards) added to the pool. These are generally installed by core drilling a hole into the deck and grouting the canopy support pole in the hole. I have yet to find any of these added canopies bonded to the grid. It is an added expense and requires extra work and knowledge to run a copper bonding conductor to a grid tie point. I have also seen cases where the core drilling will actually sever the bonding conductor running around the pool. This causes major breaches in the bonding grid. A solution is to have an electrical inspection to assure that these new items are added to the grid when a permit is pulled. Canopies are but one example of the many items that can be added or moved around any pool.

Cutting or Repairing the Pool Deck

It is common to see major sections of the concrete deck cut and replaced around a pool complex. These cuts are usually required to repair buried water leaks, add new piping, repair crumbling concrete, and so forth. A high majority of this activity will sever the bonding conductor around the pool. Remember, the bonding grid is a passive system that only functions and carries current when voltage gradients enter the complex. When cut, there are no pyrotechtronics, flashover, explosion, or overcurrent protection device operating. The person cutting the deck will usually see the 8 AWG or larger bare copper conductor (as required by Code) embedded in the concrete and think, "”must have been a scrape piece of copper just laying in the concrete when the deck was poured.”" I have yet to see a new bonding conductor added to replace the one that was cut. The result is a breached bonding system.

The Solution for Safety

How can the engineer/designer, contractor, owner, and inspector make sure this passive bonding system is installed correctly and maintained? Each party working in this environment must be familiar and trained on the requirements of 680.26. The contractor should tie every metallic part within five feet of the inside walls of the pool to the common bonding grid. This must be done with clamps, pressure connectors, or exothermic welding suitable for the purpose as outlined in 680.26(C) and also covered in 110.3(B). The bonding methods and materials must be approved for the aquatics environment (see 100.11). The only suitable materials per Code are copper, brass, copper alloys, and stainless steel. Steel items deteriorate quickly around chlorinated water and are not acceptable. Inspectors should view the bonding system at several points in time during its installation. Bonding conductors and connections must be looked at before the concrete decking is installed and covers these items. All items in the installation should be labeled and listed for this environment. Filter houses need to be viewed as well. All circulating pump motors need to be connected to the bonding grid unless double insulated. An ohmmeter is helpful to assure the bonding is done at a low impedance level. A reading of under 2 ohms should be present between all required items. (Ed. note: This is a recommendation of the author and not a provision of the Code. Even though this is a good value to shoot for, there is nothing mentioned in the Code about required testing or maximum resistance values between metallic parts of the common bonding grid.)

Third Party Certification of Bonding Grid

Because of the complexity and passive nature of the bonding grid, some consulting engineers are requiring a third party to come in and inspect, test, and certify that the aquatics complex bonding grid has been installed per specifications. This certification occurs before the aquatics complex is allowed to open. This final stage can assure that this passive system is completely intact as designed and functioning to provide a degree of safety to the patrons and employees at the local pool.

The Final Word

Nothing can substitute for training and experience. Those designing, installing, and inspecting aquatic parks, pools and similar installations must be familiar with Article 680. Detailed requirements are included in this article on all facets of electrical installations in this environment. Proper design, installation, inspection, and certification of the pool bonding grid system can assure that the swimming experience at the pool is both refreshing and safe.


Read more by Bob Herzig

Tags:  Featured  November-December 2002 

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