Current Transformer Burden

In CTs, the secondary has very small impedance referred to as burden, so the CT practically operates on short circuit conditions. The burden for CT is the volt-ampere (VA) loading which is imposed on the secondary at rated current. The burden can also be expressed as the ratio between secondary voltage and secondary current. 

A metering CT has lower VA capacity than a protection CT. A metering CT has to be accurate over its complete measuring range. Such a CT’s magnetising impedance at low current and hence low flux should be very high. 

The magnetising impedance is not constant for a CT’s operating range due to the non-linear characteristics of the B-H curve. It cannot give linear response during large fault currents. For protection CT, linear response is expected for up to 20 times the rated current. It is also expected to give precise performance in the normal operating currents up to high fault level currents.

Common Ingress Protection (IP) Ratings for Electric Motors

Enclosures of electrical equipment, per characteristics where they will be installed and their maintenance accessibility, should offer a certain degree of Protection also known as Ingress Protection (IP). Standard IEC 60034-5 defines the degrees of protection of electrical equipment by means of the characteristic letters IP, followed by two characteristic numerals. NEMA also defines IP ratings for enclosures.

Motor	Degree of Protection	First Characteristic Numeral		Second Characteristic Numeral
		Protected Against Accidental Contact	Protected Against Solid Objects	Protected Against Water
Open Motors	IP00	Non-Protected	Non-Protected	Non-Protected
	IP02	Non-Protected	Non-Protected	Protected against dripping water even when tilted 15° vertically
	IP11	Protection against accidental contact with the hand	Ingress of solid objects exceeding 50mm in diameter	Protection against dripping water falling vertically
	IP12	Protection against accidental contact with the hand	Ingress of solid objects exceeding 50mm in diameter	Protected against dripping water even when tilted 15° vertically
	IP13	Protection against accidental contact with the hand	Ingress of solid objects exceeding 50mm in diameter	Protected against dripping water even when tilted 60° vertically
	IP21	Protection against the touching with the finger	Ingress of solid objects exceeding 12mm in diameter	Protection against dripping water falling vertically
	IP22	Protection against the touching with the finger	Ingress of solid objects exceeding 12mm in diameter	Protected against dripping water even when tilted 15° vertically
	IP23	Protection against the touching with the finger	Ingress of solid objects exceeding 12mm in diameter	Protected against dripping water even when tilted 60° vertically
Closed Motors	IP44	Protection against the touching with tools	Ingress of solid objects exceeding 1mm in diameter	Protection against splashing water from any direction
	IP54	Protection against contacts	Protection against accumulation of harmful dust	Protection against splashing water from any direction
	IP55	Protection against touches	Protection against accumulation of harmful dust	Protection against any water jets from any direction

For special and more dangerous areas where electric motors are required to be applied, the following degrees of protection are commonly used:

IPW 55 (Weather protection)

IP56 (Protections against water jets)

IP65 (Totally protected against dust)

IP66 (Totally protected against dust and water jets).

What is a Motor Service Factor?

Motor Service Factor (SF) is the percentage multiplier that a motor can handle for short periods of time when operating within its normal voltage and frequency tolerance. In other words, it is a fudge factor that give extra horsepower when it's occasionally needed.

For instance, this 1/2 horsepower shown in the photo has a service factor of 1.25 so it can actually output 25% more power required for short periods of time. This comes in handy if the density of the liquid increases or a higher than normal flow rate is required.

Fractional horsepower motors usually have a higher service factor up to 1.5 since their power consumption does not lead to significantly higher winding temperatures. Motors of 10 hp and up usually have a service factor of 1.15.

The Canadian Electrical Code defines service factor as a multiplier that, when applied to:

• The rated horsepower of an AC motor,
• To the rated armature current of a DC motor, or
• To the rated output of a generator,

... indicates a permissible loading that may be carried continuously at rated voltage and frequency. Note how the CEC allows for continuous operation while we reserve it for only short periods of time to ensure reliability.

As electrical consultants, using the service factor gives us a margin of safety that allows our design to:

• Extend the life of the motor by lowering the temperature of the insulation winding.
• Compensate for low or unbalanced supply voltages.
• Accommodate the variability in horsepower. A 15% buffer is a nice margin to have especially for those occasional overload conditions.

Why Have a Service Factor

Operating a motor at its limit makes it more prone to overheating. A service factor allows the motor to operate below its theoretical maximum so it can run continuously with a cooler winding temperature at rated load. This leads to a longer life and better reliability.

Designing with the Service Factor

We never design systems to operate continuously at the maximum level (redundancy is a different topic). It is good practice to size a motor for continuously operation that is below the service factor percentage. An Electrical load monitoring test help to see how efficient a motor is running.

Operating in the service factor area may result in:

• Decreased efficiency (more energy usage).
• Decreased power factor (more reactive power usage).
• Overheating and damage to the wire insulation.
• Incorrect starter sizing which may lead to inadequate starting & pull-out torques.

A motor services factor is a margin of safety which increases the reliability of building systems.