How Voltage Drop Affect Electrical Equipment and Installations

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In an electrical installation, determination of voltage drop from the point of supply to the load is very important. Excessive voltage drop on the supply line impacts most electrical equipment but not to the same degree. The effect of voltage drop on some key electrical equipment is given below.

Effects of Voltage Drop
Electric Motors
In an electric motor, the torque is proportional to the square of the supply voltage. Therefore, if the voltage drops the starting torque will also decrease, making it more difficult to start up motors. The maximum torque will also decrease .

Incandescent Lamps
The more the voltage drops the weaker the beam becomes and the light takes on a reddish tone.

Discharge Lamps
In general, they are not very sensitive to small variations in voltage, but in certain cases, great variation may cause them to switch off.

Electronic Appliances
They are very sensitive to
variations in voltage and that is why they are often fitted with internal stabilizer circuitry.

Electromechanical Devices
Devices such as contactors and auxiliary releases have a minimum voltage below which their performances cannot be guaranteed. For a contactor, for example, the
holding of the contacts becomes unreliable below 85% of the rated voltage.

Voltage Drop Limits for Electrical Installations
To limit the problems of voltage drop on electrical equipment, the IEC 60364-5-52 “Electrical installations of buildings. Selection and erection of electrical equipment - Wiring systems” Annex G states that the voltage drop between the origin of an installation and any load point should not be greater than certain limits specified in the table below expressed with respect to the value of the nominal voltage of the installation :

Type of Installation Lighting (%) Other Uses (%)
A - low voltage installation supplied directly from a public low voltage distribution system 3 5
B - Low voltage installation supplied from a private low voltage supply. 6 8
1. As far as possible, it is recommended that voltage drop within the final circuits do not exceed those recommended for type A installations.
2. When the main wiring systems of the installation are longer than 100m, these voltage drops may be increased by 0.005% per meter of wiring system beyond 100m without this supplement being greater than 0.5%
3. Voltage drop is determined from the demand by the current using equipment, applying diversity factors where applicable or from the values of the design current of the circuits

Exceptions to the IEC 60364-5-52 standard
A greater voltage drop may be accepted for
  • Electric motors during starting period
  • For other electrical equipment have high in-rush current
Provided that in both cases it is ensured that the voltage variations remains within the limits specified in the relevant equipment standard

The following temporary conditions are excluded from the voltage drop restrictions in the table above:
  • Voltage transients
  • Voltage variation due to abnormal operation