A Website on Maintenance of Electrical Systems

Installation and polarity of a current transformer. Since current transformers are used on AC circuits and the direction of the current in an AC system is constantly changing, the polarity of the transformer terminals also changes on each cycle. One might easily think that current transformers like most AC devices have no polarity. If you follow this assumption, you would be wrong because transformers have polarity. The Polarity of a CT is very important when installing them for relay protection and metering. The polarity of a CT is determined, by the direction in which the coils are wound, around the CT Core, and how the secondary leads are brought out of the transformer case. However, whenever we talk about the polarity of a transformer, we are referring to the instantaneous polarity. In essence, the polarity markings on the current transformer denote the instantaneous polarity of the transformer. The easiest way to understand this is by using the dot convention. From the diagram b

A current transformer is a type of instrument transformer used to step down large values of current on the primary side to a manageable level of secondary current proportional to the amount of current flowing through the primary. When you see a CT with 800:5 ratio, it means the circuit is operating in the range of 0 to 800 Amps on the primary side and it would step the current down proportionally to the range 0 to 5 Amps for the devices on the secondary side. The output from the secondary side of a current transformer is usually used for operating measuring devices like ammeters and protective devices like relays. The current transformer provides electrical isolation for the measurement and protective devices on the secondary side as these devices are insulated away from the high-voltage and current on the primary side. Working Principle The working principle of the current transformer is the same as that of the voltage transformer it contains a primary and a secondar

It is very important to measure the resistance between the grounding rod and “true Earth”. Every ground rod must prove to have a low resistance to the earth. Different methods are used to test and confirm soil resistivity to ensure that it meets NEC 250.56 requirements. The most common methods used in the industry are listed below. 1)        3-point or Fall of potential Method, 2)        4-point Method or  Wenner method, 3)        Clamp-On Method. 3-Point Method – This is the most common method used when measuring the earth's resistance of a ground rod. It is used to effectively measure the ability of the ground rod to dissipate energy from a site.  Three electrodes used in this test. the three electrodes are; The ground rod Under Test, The Voltage electrode (P) and the Current electrode(C) The ground rod under test must first be disconnected from the earth bus, then the auxiliary test electrodes would now be driven into the ground in a straight line away f

Bonding  is the “permanent joining of metallic parts to form an electrically conductive path that ensures electrical continuity and the capacity to conduct safely any current likely to be imposed.” All Metallic structures in a facility that can be energized due to an earth or ground fault, static electricity buildup or lightning strikes are bonded together then connected to the electrical system earth point. This practice is called Equi-Potential Bonding.  In some installations, the bonding loop for lightning protection is separate from the bonding loop for electric motors, cable trays, metallic pipes, and steel structures. The electrical system earth point is the point where the grounding wires are physically connected to the earth with the aid of a grounding electrode.  The grounding electrode generally  stabilizes voltages from the power source  as in the case where the neutral of a transformer is bolted to ground and driven into the earth.  the grounding electrode