13 Basic expressions often used in electrical testing

Testing of electrical installations

This is the simple list of basic terms you can often hear when testing and measurements of electrical installation (in general) is being performed. While expirienced electrical engineers will find this list short, I hope beginners will catch the essence and continue exploring this field of electrical engineering.
Feel free to suggest me an expression (along with description) you think it should be listed, it will be my pleasure to add it to the list and to move away from number 13

Ok, so here is the list:

1. Active accessible conductive part
2. Passive accessible conductive part
3. Electric shock
4. Earthing electrode
5. Nominal voltage
6. Fault voltage
7. Contact voltage
8. Limit Contact voltage
9. Nominal load current
10. Nominal installation current
11. Fault current
12. Leakage current
13. Short-circuit current

1. Active accessible conductive part

Active accessible conductive part is the conductive part of an electrical installation or appliance such as the housing, part of a housing etc. which can be touched by a human body. Such an accessible part is free of mains voltage except under fault conditions.

Switchboard contruction grounded (photo credit: ecsanyi)

2. Passive accessible conductive part

Passive accessible conductive part is an accessible conductive part, which is not a part of an electrical installation or appliance, like:

• Heating system pipes,
• Water pipes,
• Metal parts of air condition system,
• Metal parts of building framework
• etc.

Equipotential bonding of metal pipes (photo credit: diy.stackexchange.com)

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How to measure insulation resistance of a motor

Winding insulation resistance

If the motor is not put into operation immediately upon arrival, it is important to protect it against external factors like moisture, high temperature and impurities in order to avoid damage to the insulation. Before the motor is put into operation after a long period of storage, you have to measure the winding insulation resistance.

If the motor is kept in a place with high humidity, a periodical inspection is necessary.

It is practically impossible to determine rules for the actual minimum insulation resistance value of a motor because resistance varies according to method of construction, condition of insulation material used, rated voltage, size and type. In fact, it takes many years of experience to determine whether a motor is ready for operation or not.

A general rule-of-thumb is 10 Megohm or more.

Insulation resistance value	Insulation level
2 Megohm or less	Bad
2-5 Megohm	Critical
5-10 Megohm	Abnormal
10-50 Megohm	Good
50-100 Megohm	Very good
100 Megohm or more	Excellent

The measurement of insulation resistance is carried out by means of a megohmmeter – high resistance range ohmmeter. This is how the test works: DC voltage of 500 or 1000 V is applied between the windings and the ground of the motor. Ground insulation test of a motor

 

During the measurement and immediately afterwards, some of the terminals carry dangerous voltages and MUST NOT BE TOUCHED.

Now, three points are worth mentioning in this connection: Insulation resistance, Measurement and Checking.

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