Assemblies of switchgear and control panels (2)

Continued from 1st part of article: Assemblies of switchgear and control panels (part 1)

Other components that are needed for the proper operation of the switchgear or control panel but not necessarily part of the installation or maybe installed remotely are:

• The batteries that supply control power for the proper operation of the circuit breakers
• Protection devices (will be decribed in next tech. article)
• Metering packages  (will be decribed in next tech. article)
• Control devices (will be decribed in next tech. article)

The Batteries (Control Power Supply)

They could be lead acid or nickel cadmium complete with a charger system sized and rated to operate all loads under normal and power loss conditions.

The stationary battery is designed to serve as an auxiliary /standby source of power to all devices connected to it. The battery is normally mounted on racks and is continuously charged except for intermittent discharging intervals of varying times and power. Battery voltage gradually declines during discharge and should not be permitted to drop below the minimum tolerated by the load plus the line drop.

The rate of voltage decline depends upon:

1. The demand current of the load,
2. Duration of the discharge,
3. Chemical design and type of cells,
4. Number and size of plates in each cell,
5. Battery state of charge at beginning of discharge,
6. Age of battery cells and temperature of cells.

The capacity of the battery is basically its ability to supply a given current for a given period of time at a given cell temperature without going below the minimum voltage (batteries are rated in ampere­hour at a given discharge rate).

Stationary batteries are usually rated for 8­ hours, 3­ hours, 1­ hour, 1­ minute discharge.

The ampere hour rating is simply the product of the discharge in amperes multiplied by the given discharge time period. In the next few paragraphs the types of plates and grid alloys for the lead acid and alkaline (Ni­Cad), the electrolyte properties, battery charging, battery safety and finally battery maintenance will be covered.

For the lead acid battery, the positive plates a vailable are: the pasted (F aure) plate which comprises of a latticework metallic grid with the openings filled with lead oxide paste. The grid may be made up of lead antimony or lead calcium, the properties of which will be given later .

The second type is the multitubular plates, which use porous plates to contain the lead o xide. The grid (lead antimony) is basically a row of spines extending from the top bar to the bottom cap bar. Porous tubes filled with lead oxide (powdered) with the grid forms the positive plate.

This design form prvides more AH of capacity per cubic foot of battery volume at moderate rates of discharge.

The third and last type is the plante type which is considered to have the longest life expectancy of all lead acid stationary battery designs. The positive plate consists of a grid (lead antimony) of large area with thin layers of lead oxide. Such plates have complex designs with circular openings where corrugated lead ribbons are rolled into spiral ribbons.The negative plates irrelevant of the type of the positve plates are built with pasted plate design. Metallic sponge lead is used on the negative plates.

The grid achieves and retains a physical shape and conducts the current to all parts of the material. Pasted plates with lead calcium alloy grids are used in sealed maintenance free lead acid cells due to the fact that this type does not require watering during its life time.

Lead antimony is preferred for installations where elevated temperature and frequent cycling is encountered. Lead calcium is also used for installations requiring longer intervals between maintenance watering.

The disadvantage of lead calcium is that under frequent cycling the life of the battery is reduced significantly.

For the nickel­alkaline batteries there are two types of plates, the pocket type and the sintered type. The pocket type is used for both positive and negative plates. The activ e material (nickel hydrate ­ positive and cadmium sponge ­ negative plus additives to help conductivity) is sandwiched between two perforated strips (nickel plated steel). The strips are crimped together and this assembly is placed in a U­ shape frame. After intermeshing the positive and negative the insulator pins are put in place, through the frame and plates.

These elements are than put in a container and the cell cover (with vent cap and appropriate hole for terminal poles) is installed.

For lead acid batteries

The electrolyte is a solution of diluted sulphuric acid. When the battery is fully charged, the positive plate is lead peroxide and the negative one is sponge lead. The specific gravity of the electrolyte is maximum at start of discharge, the reaction between the acid and the active material produces lead sulphate and water the specific gravity gradually decreases.

When the battery is placed on charge the reverse takes place. The volume of acid in the electrolyte of a lead acid battery is measured by specific gravity.

Specific gravity for stationary batteries used for switchgear, control and emergency lighting is approximately 1.210 (the highest volume of acid as compared to other applications but in lower concentration).

To determine the state of charge of the battery, the gravity reading is compared with the full charge value published and to the specific gravity drop of a particular cell size at a specific discharge rate. The reading has to be corrected to the ambient temperature at time of measurement if other than 25 deg C.

For the Ni­Cad Batteries

The electrolyte is a solution of potassium hydroxide diluted in water with normal specific gravity of 1.16 to 1.19 at 25 deg C, additives are added to improve its capacity. When the battery is fully charged the positive plate ­ nickel hydrate is highly oxidized and the negative plate is sponge metallic cadmium. After discharge takes place the positive plate reduces to lower oxide while the metallic cadmium in the negative plate oxidizes. None of the constituents of the alkaline electrolyte combine with the active material of the plates during charging or discharging.

The specific gravity of the electrolyte can not be used to indicate the state of charge of the battery. The specific gravity readings will vary from normal rating when he electrolyte temperature is lower or higher than 25 deg C, when the solution level drops below the normal, or the battery has been in service for long time.

In this paragraph charging of both types of batteries will be covered. The lead acid stationary batteries are continuously float charged while on a standby status and are charged with a high constant potential current­ limited level after a discharge. The higher voltage value depends on the recharge time required to recharge the battery. As the charging time is shortened the charger’s cost increases.

The charger is a static rectifier (SCR), its function is to change the single phase or three phase input (120, 208, 240, 480, 600V ­ 60HZ) to a d.c. output suitable for charging the battery and maintaining a constant voltage throughout the battery’s load range. To prevent the self discharge phenomena (standing loss) the charger maintains a float charge that continuously monitors and corrects for these internal losses.

Batteries equipped with lead calcium grids rather than lead antimony require a float charge per cell higher than its counter part (eg. 2.2V vs. 2.15V).

Current drawn under different ambient temperature conditions is as follows: the higher the temperature the higher the current drawn and vice versa. Chargers compensate, usually, for variations in float­current demands. The floating current is directly proportional to the cell (battery) voltage. The faster the rate of change in voltage the higher he current drawn at the floating voltage.

The charger should be able to provide the f loating v oltage with a v ariation of no more than plus or minus 1% throughout its ampere rating.

Chargers for Ni­Cad batteries (stationary) are simillar to the ones used for lead acid, float charged while on standby and a higher charge after a discharge (for a prolonged period) after which float charge level is resumed until further discharge. The value of the higher charging current is at least 5% higher than the 8 ­hour discharge capacity. Float rate for Ni­Cad batteries is about 1.4 to 1.45V per cell at 25 deg C.

After a discharge the higher charge is applied until voltage rises to 1.6 per cell (it is maintained for 15 to 30 hours). The size of the charger ampere rating is usually matched to the load demand plus the maximum high charging rate required by the battery.

The battery starts to discharge when the demand exceeds the charger supply or when the a.c. supply to the charger is cut off.

The data required for the proper selection of a charger are:

1. Total current,
2. Voltage (d.c.)
3. Duty (continuous, standby , combination)
4. Automatic control (recommended)
5. Voltage (a.c.) and number of phases

Standard components that are found in a charger are:

1. A.C. & D.C. fuse protection
2. D.C. ammeter and voltmeter
3. A.C. on pilot lamp
4. Reverse battery protection
5. A.C & D.C. surge protection
6. “High rate” on pilot light
7. Automatic current limit short circuit
8. Float / high rate current limit potentiometers
9. Alarm circuits
10. High rate charge timer

A storage battery is constantly live electrically and therefore a source of electrical shock. Tools should never be laid on top of the battery as such an action can cause severe short circuits.

Refer to the local electrical safety code (for example CSA C22.1) for requirements of the method of connecting the batteries and of the battery room ventilation.

Electrolyte used in lead acid batteries is highly corrosive. If spilled on any object it has to be neutralized through the use of 1 lb of bi­carbonate soda and 1 lb water. Electrolyte used with alkaline batteries, if spilled has to be neutralized by flooding the spill with solution of vinegar diluted with water 5050 proportion.

It is highly recommended to use goggles (or face shield) and rubber gloves when maintenance is performed on a battery. Maintenance to a battery is the proper addition of water (tap or distilled) to correct the electrolyte solution level and to keep a record sheet with pertinent information. Tap water can be used if the impurity limits are not exceeded.

It is important to keep the battery clean and dry, the connectors tight, the electrolyte at the proper level, the electrolyte density according to the supplier recommendations.

To be continued…

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