ELECTRICITY UNITS, SYMBOLS AND LAWS

Remark

A unit preceded by a "k" is multiplied by 1000.

(e.g. 1 kW = 1000 W).

Basic laws

I = U/R

P = U x I

Connection in parallel (connection + to + and - to -)

U = U1 = U2 = …

I = I1 + I2 + …

P = P1 + P2 + …

Connection in series (connection + to - and - to +)

U = U1 + U2 + …

I = I1 = I2 = …

P = P1 + P2 + …

Relation between W and VA

As a rule of thumb, 1 VA = ± 0.8 W

ELECTRICAL POWER TRANSFORMING DEVICES

Apart from the difference between AC and DC, a source of electricity can take various forms. Below is an overview of the devices which can modify the form of the power supply.

Transformer

Mains voltage differs from country to country. Europe uses 230 or 240 V AC, the USA uses 110 V AC, and some other countries use 127 V AC. In order to use a device which is not designed for the locally available voltage, a transformer is needed. Please note that the frequency in the USA’s (and a couple of other countries) can be 60 Hz. All the equipment in this catalogue is primarily compatible with the ‘normal’ 50 Hz.

AC/DC adaptor (or "power supply")

Many electrical devices (laptop computers, HF or VHF radios etc.) run on DC within a range of 3 to 20 V. These devices can be powered from the mains by means of an adaptor which converts AC to the required DC voltage. Such an adapter is also called an AC/DC converter or power supply, and is usually supplied together with the device.

Battery charger

A battery charger also supplies DC from an AC source, but the DC current and voltage are suitable for charging a battery.

Caution: even though both supply DC current from an AC source, an AC/DC adaptor and a battery charger are never interchangeable!

A good quality charger will have 3-stage automatic charge management (boost,absorption, float):

• During the first stage (boost), the charger supplies the maximum current allowed by the battery (0.2 to 0.5 A per Ah capacity, depending on the battery type) until the voltage reaches 14.5 V (for a 12 V battery). This maximum current defines the current rating (in A) of the charger to be installed. This maximum current may be adjustable to enable different batteries to be charged by the same charger.
• During the second stage (absorption), the voltage is maintained at 14.5 V, while the current slowly decreases.
• The third stage (float) begins once the current drops below 0.5 A. Then the voltage applied by the charger is reduced to 13.6 V. At this point, the battery is supposed to be fully charged. The voltage applied by the charger maintains the charge and it supplies the low current that is necessary to do so.

A good quality charger must also be equipped with a temperature sensor. If the voltage is too high for a given temperature, the battery will slowly deteriorate by losing water, which is irreversible in the case of a sealed battery. Charging voltage must be adjusted to battery temperature (decrease of ± 0.025 V per increase of 1 ºC). If the temperature sensor is installed, this voltage correction is automatic.

DC/AC inverter

A DC/AC inverter produces an AC output from a DC input. This is the only way to get AC when other sources (mains, generator) are not available. The DC source usually consists of batteries that have to be charged either from AC sources through a battery charger, or from DC sources (solar panels etc.) equipped with a charge regulator.

DC/DC converter

A DC/DC converter enables a DC voltage to be converted into another DC voltage. This kind of converter is useful for example to obtain 12 V DC on board a truck which has a 24 V DC electrical system.

Solar panels

The production of DC electricity from sunlight is clean, quiet, requires little maintenance and is durable. The energy produced daily by a solar panel will depend on the location's mean daily solar irradiance. This information is essential to size an installation: it will vary accordingly to the season. It is expressed in kWH/m²/day.

Solar panels are rated in Wp (watt-peak). Their actual production when the sun is bright is approx. ¾ of that figure. They often come in 12 volt and 24 volt panels.

They will often be used to charge batteries. You will have to use a regulator in-between the battery and the panel. We now only supply so-called MPPT regulators that give the best results.(Maximum Power Point Tracking)

MPPT regulators can convert panel voltages and can be used for various battery voltages (12, 24 or 48 volt).

ELECTRICAL PROTECTION DEVICES

1. Protection against overloading

The main protection devices in an electrical circuit are fuses or thermal-magnetic circuit breakers. These devices protect the installation against overloading or short circuits by cutting off the supply if the current gets too high. The value of an overload protection device is given in amperes and indicates the maximum current that can be permanently allowed into the circuit.

Fuses

Fuses consist of a cartridge containing a metal wire that melts when the current reaches a defined value for a given time. They can be used once only and must be replaced once they have blown. Fuses come in different types (slow, normal, or fast) and must be chosen according to the device or circuit to be protected.

Thermal-magnetic circuit breakers or MCB

Thermal-magnetic circuit breakers (MCBs) combine thermal and magnetic protection.

The thermal protection causes the device to trip in case of overloading. The trip time varies according to the size of the overload.

The magnetic protection causes the device to trip instantaneously in case of a short circuit.

There are three main types: B, C and D-curve.

B-curve: The circuit breaker has a relatively low magnetic trip (between 3 and 5 xIn) and eliminates short circuits of very low value. This curve is also used for circuits having long cable lengths, in particular in TN mode.

C-curve: This circuit breaker covers a very large majority of needs (inductive receivers) and is used especially in domestic electrical installations. Its magnetic trigger is between 5 and 10 xIn. This is the most commonly used curve in MSF.

D-curve: This curve is hardly used. It could be used for the protection of circuits where there are very high current peaks at power-up. The magnetic trip of this circuit breaker is between 10 and 20 xIn.

Thermal-magnetic circuit breakers can be manually reset after tripping.

Circuit breakers are installed at the head of an electrical installation (distribution box or board) and protect each circuit individually. They are selected according to the cross-section of the electrical cabling to be protected.

2. Protection against AC supply faults

There are 3 types of AC disturbance that can damage connected equipment.

• Voltage variations: The voltage supplied is too low (brownout), too high (surge) or fluctuates too much.
• Voltage spikes: The supply exhibits brief high-voltage pulses.
• Power cuts: The supply is intermittent, fuses have blown, or the generator tank is empty…

Voltage limiter

An electrical appliance will work normally as long as the input voltage remains within a certain range called the "input tolerance". Most electrical appliances intended for 230 V AC have an input tolerance that ranges from 185 to 250 V AC. These appliances should be protected by a voltage limiter which interrupts the supply when the voltage goes beyond the tolerance range. MSF uses voltage limiters on most electrical appliances.

Lightning protector or surge suppressor

Most electrical appliances are quite vulnerable to voltage spikes.

During a thunderstorm, these spikes can reach several thousand volts and cause significant physical damage. A lightning protector or surge suppressor is a protection device that can eliminate surges and divert the current to earth. Contact your technical department for selection and installation of a lightning protector or surge suppressor.

Voltage regulator/stabilizer

Appliances which have a narrow input tolerance will not be properly protected by a voltage limiter when the input voltage goes outside the tolerance range.

Such appliances need to be protected by a voltage regulator which takes an input voltage that is too low, too high or too fluctuating and corrects it.

Some protection devices (called voltage stabilizers, constant voltage transformers or line conditioners) combine the functions of voltage limitation and regulation, but as there are no standard designations or specifications it is extremely difficult to select the right device without testing it.

MSF has selected a voltage regulator/stabilizer that incorporates all the above mentioned protection functions.

Uninterruptible power supply (UPS)

Power cuts do not pose a problem for some appliances (e.g. lighting). But for other devices (e.g. computers), power cuts can cause loss of data. These devices need to be protected by an offline UPS.

A UPS combines a battery, a battery charger and a DC/AC inverter into one device. It provides an uninterrupted 220 V AC power supply and gives a limited independent operating time in the event of a power cut.

The commercially available UPSs are designed for desktop computers. They provide around 10 minutes of power after a power cut, but they do not protect against voltage spikes.

A special UPS is included in the catalogue for critical/essential medical and laboratory equipment. This is referred to as double conversion (also called online) UPS. This UPS is primarily intended for protection of the equipment.

Power supply module

If the mains supply is of very poor quality and/or the appliances to be protected require significant independent operating time, it is better to instal a power supply module. This module combines the functions of a battery, a battery charger, a limiter and an inverter:

• Energy storage in the batteries
• 12 V DC supply
• 230 V AC supply
• AC regulation and stabilisation by double conversion (AC=>DC=>AC)
• UPS: uninterrupted AC power supply with ample independent operating time

3. Protection against insulation faults or shocks

Besides earthing, there is one protection item very important for safety: the RCD or Residual Current Device, also know under a number of other abbreviations like RCBO, ELCB, RCCB RCTD. The RCD measures if there is ‘leakage current’ and if over 30 mA , it will switch off. RCD’s work correctly with earthed but also with not-earthed systems. Quality is an issue with local purchase so only buy from accredited suppliers (ask your support department).

Electrical consumption

INGRESS PROTECTION RATING (IP)

Ingress Protection (IP) ratings are developed by the IEC for specifying the environmental protection the enclosure provides.

The IP rating has two numbers:

1. Protection from solid objects or materials

2. Protection from liquids (water)

Example - IP Rating

With the IP rating IP 54, 5 describes the level of protection from solid objects and 4 describes the level of protection from liquids.

An "X" can used for one of the digits if there is only one class of protection, i.e. IPX1 which addresses protection against vertically falling drops of water e.g. condensation..