EETC Electrical Equipment & Maintenance 4

Question 1

What is the recommended maximum shaft voltage level on a VFD-driven motor before bearing damage becomes a concern?

Accepted Answer

500 mV peak

Answer

Shaft voltages above approximately 500 mV peak can discharge through bearings, causing pitting and fluting damage known as electrical discharge machining (EDM).

Question 2

In a motor control center (MCC), what is the purpose of the stab assembly?

Accepted Answer

Make electrical connection between the bucket and the vertical bus bar

Answer

Stab assemblies are the plug-in connectors on MCC buckets that make contact with the vertical bus bars when the unit is inserted into the structure.

Question 3

An overload relay is tripping on a motor that is lightly loaded. The MOST likely cause is:

Accepted Answer

Incorrect overload relay heater elements installed for the motor FLA

Answer

Heater elements sized for higher FLA will not trip under normal load, but elements sized too small will trip the relay even under light load conditions.

Question 4

What does the term 'dielectric withstand' (hi-pot) testing verify in electrical equipment?

Accepted Answer

The ability of insulation to withstand overvoltage without breakdown

Answer

Hi-pot testing applies elevated voltage to insulation to verify it will not break down under transient overvoltages encountered in service.

Question 5

When performing contact resistance testing on a circuit breaker, results significantly higher than manufacturer specs indicate:

Accepted Answer

Pitted, burned, or corroded contacts that increase resistance and heating

Answer

High contact resistance causes excessive I²R heating under load, which can cause contact failure, welding, or nuisance tripping.

(EETC) Electrical Technician Certification Practice Test

(EETC) Electrical Technician Certification Practice Test

EETC Electrical Equipment & Maintenance 4