El proceso ecográfico: Criterios de calidad

Danger of High Voltage and Current

The current due to the high voltage is extremely dangerous and could be fatal to the human. The human body acts as the conductor of electricity and when the amount of electricity flowing through the human body exceeds 9-12 mA range, fatal consequences can be observed. The amount of current through body depends upon the resistance of the body and the applied voltage. The body resistance of a healthy and cheerful fellow may be above 3 kilo Ohms under normal condition. In this case the single battery or module voltage may not be fatal when touched. But if the body is moist and the mental condition is worse, the resistance may drop significantly and even the low voltage as that of single 12 V battery may be fatal when touched. The effect of current flow in the body also depends upon the path of current. For example, touching one of the wires by only one hand may not be that critical as there is no complete path for the current to flow. But if positive terminal of the battery is touched by one hand and the negative by another, the circuit for the current is complete through the chest and the effect would be highest. The

effect of electric shock would be fatal independent of the body conditions, if the system voltage were relatively high.

The users of this manual are advised to consult other literatures on electrical safety for detail understanding of the effect of electric shock and ways to minimize it.

Common Risk Associated with Battery Bank

Battery storage in PV systems poses several safety hazards:

 Hydrogen gas generation from charging batteries

 High short-circuit currents

 Acid or caustic electrolyte

 Electric shock potential

 Hydrogen Gas

When flooded, non-sealed, lead-acid batteries are charged at high rates or when the terminal voltage reaches 2.3-2.4 volts per cell, the batteries produce hydrogen gas. Even sealed batteries may vent hydrogen gas under certain conditions. This gas, if confined and properly not vented, poses an explosive hazard. The amount of gas generated is a function of battery temperature, the voltage, the charging current, and the battery bank size. The large numbers of batteries in smaller or tightly enclosed areas require venting.

A catalytic recombiner cap (Hydrocap) may be attached to each cell to recombine some of hydrogen with oxygen in the air to produce water. If these combiner caps are used, they will require occasional maintenance.

In no case should charge regulators, switches, relays, or other devices capable of producing an electric spark be mounted in a battery enclosure or directly over a battery bank. Care must be exercised when routing conduit from a sealed battery box to a disconnect. Hydrogen gas may travel in the conduit to arcing contacts of the switch. As a safety measure smoking and use of matches, cigarette lighters or any other flaming lamps in the battery room must be avoided completely.

 High Short-circuit Current

Batteries are capable of generating tens of thousands of Amperes of current when short-circuited. A short-circuit in a conductor or in the load side not protected by over current devices can melt wires, wrenches or other tools, battery terminals, and spray molten metal around the room. The short-circuit may also ignite fire in the battery room.

Therefore the exposed battery terminals and cable connections must be protected. This generally means that the batteries should be accessible only to a qualified person. The danger may be reduced if insulated caps or tape are placed on each terminal and insulated tool is used for servicing and installation. The battery voltages must be less than 50V in dwellings. Moreover, batteries should not be installed in living areas.

 Acid or Caustic Electrolyte

A thin film of electrolyte can accumulate on the tops of the battery and on nearby surfaces. This material can cause flesh burns. It is also a conductor and in high-voltage battery banks poses shock hazard.

Battery servicing hazards can be minimized by using protective clothing including face masks, gloves, and rubber aprons. Self-contained eyewash stations and neutralizing solution would be beneficial additions to any battery room. Water should be used to wash acid or alkaline electrolyte from the skin and eyes.

Grounding and Lightning Protection

Grounding in electrical circuits is primarily for safety. A short-circuit, also known as a fault condition, occurs when an electrical conductor makes contact to any structural component ( junction box, load center, appliance, metallic conduit etc.), or to another conductor. Current can flow through this new path, which might include a person’s heart if they are touching the faulty component.

Dangerous currents can be made to bypass the body by having a highly conductive path back to the energy source in the form of a grounding wire. A large current can flow through the wire, large enough to cause the overcurrent devices (fuses or circuit breakers) to open the circuit and stop the flow. The ground wire is not a part of the power circuit. It is normally a non-current carrying wire and is on “stand-by” in case the normal circuit becomes unsafe.

All exposed metal surfaces. Including the module frames, array mounting structures, any metal housing of regulators or load centers, and all loads should be grounded. It is the general practice (unless otherwise agreed) to connect negative terminals of all the sources of electrical energy to the ground terminals. Therefore the negative legs of battery bank and array should also be grounded. This is often called “system ground”. All grounds, structural and electrical, should come together and be connected to one ground rod. More detailed discussion on selection of grounding conductor, grounding rod and grounding process can be found on the respective literature.

The basic concerns of lightning protection are to attract lightning strikes away from the installation structures and to an air terminal of adequate size and conductance to carry away the current of the strike without harm to installation or people. The basic elements of a protective system are an air terminal (arrestor), a down conductor to carry the current to earth, and a grounding system that adequately and safely dissipates the sudden current into earth.

Experiments have indicated that a glass front module with a rigid metal frame is able to withstand almost any lightning strike if the frame is grounded to the earth. The metal frame acts as a grounding rod (lightning arrestor) attracting all the current to it. However, a strike, which breaches the integrity of encapsulation, might, under some conditions, eventually destroy the module. But even if the module survives a lightning storm, additional protection may be required for the cabling, regulators and load. A grounding rod (or the frame connected to the grounding system) will protect the array from direct strikes but further protection is required for induced voltages and the side flashes they can create. The terminals of balance of system components can be protected by the use of varistors (MOV) or glass discharge tubes (GDT).

The degree of protection needed for any particular installation will be determined by factors such as site location (frequency and intensity of lightning strikes), size of the array, safety considerations, and cost.