Electrical-test@abufaris.com

As well as the level of voltage, charge or current and the nature of the environment, there are a number of other factors that need to be considered when you are assessing the risk of injury arising from electrical testing work. A risk assessment should be carried out before testing begins, to help you identify the precautions you need to take. Some questions to ask when carrying out the risk assessment are:

(a) Can the work be done with the equipment dead or energised at a safe voltage or current?
(b) Is it absolutely necessary for someone to be working on or near to equipment that is live at dangerous voltages or current levels?
(c) What is the maximum voltage on conductors that will be exposed during the work activity?
(d) Are the testers competent? Are they adequately trained and knowledgeable to do the particular work and ensure that others are not put at risk?
(e) If testers are not considered fully competent, are they adequately supervised?
(f) What physical safeguards should be applied to the equipment under test to prevent injury, eg the use of temporary or permanent screens?
(g) Is the test instrumentation of safe design? Has it been properly maintained?
(h) Is it necessary to set up a permanent test area separate from the rest of the workplace, where equipment can be taken for testing? Is it necessary to set up a temporary test area around the equipment?
(i) Are the testers able to supervise the working area sufficiently and at all times to prevent danger to others?
(j) Where testing is part of an ‘after sales service’ how much must be done at customers’ premises? If testing is being done in a customer’s home, what special precautions are required to protect the tester and others?
(k) To what extent should the testers be supervised or accompanied?
(l) If the testers design, manufacture or use any special test equipment, does it meet the safety requirement of statutory body eg. BS EN 61010-1?
(m) How big is the unit under test and how much space is required around it to undertake the testing in a safe and unconfined manner?
(n) Are all the other workshop employees competent to avoid danger if there is a need for them to approach the equipment? If not, how can you make sure that they do not do so?
(o) Will the equipment be left unattended while live, for example while being ‘soak tested’?
(p) Does the workbench or separate area require a warning, eg a light, to show that testing is in progress?
(q) Is there a need for additional emergency switching devices for use by other employees to reduce the degree of injury to testers? Can residual current devices (RCDs) be used to provide supplementary protection?
(r) Is it possible to reduce the number of available paths to earth to reduce the likelihood of a phase-to-earth shock, eg by the use of barriers, screens and insulating mats?
(s) Is it possible to use unreferenced supplies, eg isolating transformers/batteries to reduce the likelihood of a phase-to-earth shock?

Injury can occur when live electrical parts are exposed and can be touched, or when metalwork which is meant to be earthed becomes live at a dangerous voltage. The likelihood of touching live parts is increased during electrical testing and fault-finding, when conductors at dangerous voltages are often exposed. This risk can be minimised if testing is done while the equipment is isolated from any dangerous source of supply, although this cannot always be done, and care must also be taken to prevent contact with any
hazardous internally produced voltages.

The most dangerous injuries are those caused by electric shock. This is because the effects of a shock are largely unpredictable and can easily lead to a fatal injury. However, there is also a risk of burn injuries resulting from arcing when conductors are accidentally short-circuited. A secondary risk can be the harm caused by a person reacting to an electrical injury, for example by falling or being traumatised by the experience.

Electric shocks occur when contact with a live conductor causes sufficient current to pass through the body to cause an injury. As a rough guide, voltages exceeding 50 V ac or 120 V ripple free dc should be considered hazardous in a dry, unconfined, non-conductive location. These voltage values must be reduced if the location is wet, confined or conductive, so where there is an adverse environment, those in charge of the work and those doing the work should be aware of the probable increase in injury risk.

In some equipment, for example microwave ovens, high voltages of several thousand volts are used and there is a very high risk of fatal injury if the exposed conductors are touched at these voltages. Injury may also be caused by currents as low as 5 mA or by stored charges.

Suitable precautions must be taken to prevent contact with stored charges in excess of 350 mJ. If the skin is pricked or cut at the point of contact, the shock current (and hence the seriousness of the injury) will be higher. Healthy skin may also become damaged at the time of contact either by the burning effect of the current or by penetration from sharp-ended conductors.

Troubleshooting and fault analysis requires a good theoretical knowledge and analytical thinking. It is not something which can be studied from books, but has to be acquired through constant troubleshooting and experimenting. However, there are guidelines that can be followed to the troubleshooting process. Basically, troubleshooting depends on the circuit complexity, on symptoms, and on the personal experience. The most common troubleshooting techniques are listed below:

Power check:

Many times a simple issue such as a blown fuse or a flat battery is the cause of a circuit malfunction. Initially, therefore, ensure that the power cord is plugged in and that the fuses are not blown. If the circuit is battery powered, make sure that the voltage level is acceptable. If a power supply rectifier is present, check the level of the voltage at the output and make sure that the circuit is powered with the correct polarity.

Visual inspection:

This inspection is part of the so-called sensory checks. Sensory checks rely on the human senses to detect a possible fault. The visual inspection of the PCB is the simplest troubleshooting technique (which is very
effective in many of the cases). The soldered joints have to be inspected thoroughly. If any doubts exist about the quality of a certain joint, it has to be re-soldered. The PCB has to be inspected visually for any burnt components.

Sometimes, components that overheat leave a brown mark on the board. They can be used as ‘starting points’ in the troubleshooting process and the reasons why they overheat have to be determined. It is bad practice simply to replace such components, without trying to find out what actually caused the component to overheat. In many cases, the reason is a faulty (or out of range) component near the failed component. It also has to be replaced.

Using a sense of touch:

Overheated components can be detected by simply touching them. However, this check has to be performed with extreme caution. The circuit has to be turned off, and some time allowed for the large capacitors to discharge. Always touch the components with the right hand only. This is important because in the case of electric shock it is less likely that the current will pass through the heart. If possible, wear insulated shoes.

In addition, care should be taken not to burn the fingers. Using the sense of touch is a very useful troubleshooting technique in circuits, where everything seems to work properly for a while, and then the circuit fails, due to overheating of a certain component. Identifying such components helps to detect the possible cause of the fault. Special freezing sprays are available, which allow instant freezing of components. If the circuit begins to operate properly immediately after the heated component is sprayed, this is an indication that this component is causing the circuit failure.

Before replacing the component, further investigation is needed to determine what caused the overheating in the first place.

Smell check:

When certain components fail due to overheating it is possible in most cases to detect a smell of smoke. This is usually the case, if the technician happens to be there at the time the accident occurred. If not, it is
usually possible to detect the failed component by visual inspection afterwards.

Component replacement:

This troubleshooting method relies mostly on the operator’s skills and experience. Certain symptoms are an obvious indication of a particular component failure. This statement is especially true for an experienced electronic technician. For example, some TV service technicians can unmistakably identify the failed component in a TV set (even before opening it), by just briefly examining the symptoms.

Component replacement is a good troubleshooting technique for an experienced electronics technician, as it saves a lot of time and money. Moreover, this technique guarantees the success of the repair, because if
enough components are replaced, eventually the faulty one will be replaced too. However, it is recommended that the amateur technician initially applies some logical thinking to the troubleshooting process.

Signal tracing:

This troubleshooting technique is not the most common one, but it is the most desirable, as it requires intelligent and logical thinking from the troubleshooter. This method is based on the measuring of the signal at various test points along the circuit.

A test point in the circuit is the point, where the value of the voltage is known to the operator. This troubleshooting technique relies on finding a point, where the signal becomes incorrect. Thus, the operator knows that the problem exists in that portion of the circuit, between the point where the signal becomes incorrect, and the point where the signal appeared correct for the last time. In other words, the operator constantly narrows the searched portion of the circuit, until he finds what caused the fault.

There are two basic approaches in conducting the signal tracing.

In the first approach, the signal check starts from the input, checking consecutively the test points towards the output. The checks are carried out, until a point when an incorrect signal is found. The second approach is to start from the output and to work backwards towards the input in the same manner until a correct signal appears.

Following these simple steps will often fix a broken components on circuit board, however if you are still having issues I can recommend Testing Electronics Components to help you learn more on electronics repair (TV, computer monitor, vcd player, etc). This guide will teach you test methods to recognize fault in electronics circuits.

Technorati Profile

1. Based on construction:

Core type: Windings surround a considerable part of the core.

Shell type: Core surrounds a considerable portion of the windings.

2. Based on cooling:

Oil-filled self-cooled: Small- and medium-sized distribution transformers.

Oil-filled water-cooled: High-voltage transmission line outdoor transformers.

Air Cooled type: Used for low ratings and can be either of natural air circulation (AN) or forced circulation (AF) type.

3. Based on application:

Power transformer : These are large transformers used to change voltage levels and current levels as per requirement. Power transformers are usually used in either a distribution or a transmission line.

Potential transformer (PT): These are precision voltage step-down transformers used along with low-range voltmeters to measure high voltages.

Current transformer (CT): These transformers are used for the measurement of current where the current-carrying conductor is treated as a primary transformer. This transformer isolates the instrument from high-voltage line, as well as steps down the current in a known ratio.

Isolation transformer : These are used to isolate two different circuits without changing the voltage level or current level.

Some points about transformers:
• Used to transfer energy from one AC circuit to another
• Frequency remains the same in both the circuits
• No ideal transformer exists
• Also used in metering applications (current transformer, i.e., CT, potential transformers, i.e., PT)
• Used for isolation of two different circuits (isolation transformers)
• Transformer power is expressed in VA (volt amperes)
• Transformer polarity is indicated by using dots. If primary and secondary windings have dots at the top and bottom positions or vice versa in diagram, then it means that the phases are in inverse relationship.

A transformer is a device that transforms voltage from one level to another. They are widely used in power systems. With the help of transformers, it is possible to transmit power at an economical transmission voltage and to utilize power at an economic effective voltage.

Transformer working is based on mutual emf induction between two coils, which are magnetically coupled.

When an AC voltage is applied to one of the windings (called as the primary), it produces alternating magnetic flux in the core made of magnetic material (usually some form of steel).

The flux is produced by a small magnetizing current which flows through the winding. The alternating magnetic flux induces an electromotive force (EMF) in the secondary winding magnetically linked with the same core and appears as a voltage across the terminals of this winding.

Typically, the coil connected to the source is known as the primary coil and the coil applied to the load is the secondary coil.

SINGLE PHASE TRANSFORMER

A single-phase transformer consists mainly of a magnetic core on which two windings, primary and secondary, are wound. The primary winding is supplied with an AC source of supply voltage V1.

The current I, flowing in the primary winding produces flux, which varies with time. This flux links with both the windings and produces induced emfs.

The emf is also induced in the secondary winding due to this mutual flux. The magnitude of the induced emf depends on the ratio of the number of turns in the primary and the secondary windings of the transformer.

The ratio of the primary potential to the secondary potential is the ratio of the number of turns in each and is represented as follows:

N1/N2 = V1/V2

A step-up transformer increases the output voltage by taking N2 >N1 and a step-down transformer decreases the output voltage by taking N2 <N1.

When the transformer is loaded, then the current is inversely proportional to the voltages and is represented as follows:

V1/V2 = I2/I1 = N1/N2

EMF equation of a transformer:
rms value of the induced emf in the primary winding is:

E1 = 4.44 × f × N1 × Ø_m

rms value of the induced emf in the secondary winding is:

E1 = 4.44 × f × N2 × Ø_m

Where:
N1 = Number of turns in primary
N2 = Number of turns in secondary
Ø_m = Maximum flux in core and
f = Frequency of AC input in Hz.

IDEAL TRANSFORMER

• No loss or gain of energy takes place.
• Winding has no ohm resistances.
• The flux produced is confined to the core of the transformer, which links fully both the windings, i.e., there is no flux leakage.
• Hence, there are no I ²R losses and core losses.
• The permeability of the core is high so that the magnetizing current required to produce the flux and to establish it in the core is negligible.
• Eddy current and hysteresis losses are negligible.