Insulation Resistance (Megger) Testing

This is EHT Groups internal procedure on how to conduct megger testings.

Video

IR Test Instructions

Insulation Resistance (IR) meters produce a voltage potential in cables and equipment to test the insulation value. Caution should be used when testing not to shock the operator or others in the area. Special permits may be required by site policy. Ensure to follow the highest regulating policy (i.e. C.E.C., Provincial, Site, Corporate). Testing will only be done by qualified workers, utilizing proper PPE, insulated tools, and safe work practices and procedures.

IR Test Equipment:

• Test meter and leads must be visually inspected before testing commences. Check meter and leads to damage, tag-out meter and return if there is visual damage.
• The meter should be calibrated
• Test meter by putting leads together and pushing test button or crank, then by ensuring leads are apart and pushing test button or crank. This test should show you the meter goes respectively from zero ohms to Mega ohms (infinity). Newer digital meters may give you a high reading of maybe 2000, or 4000 Mega ohms, depending on voltage setting. Ensure you know the operation, and the output capability of the meter and that it is rated for the cabling or equipment you are testing.

Personal Protective Equipment (PPE):

 Ensure proper PPE is available, such as, flagging tape and tags, gloves, face-shield, ground equipment. Reference the MSCP’s ‘Electrical Practices’ for proper PPE selection.

Cables/EHT

• Caution must be used when working around potentially energized cables, properly identifying the cable at each end can sometimes be difficult.
• Follow safe lock out procedure if required.
• Follow safe work practices and procedures
• Red tape area around cable end, and tape exposed wires, if not monitored, to protect others from electrical shock.

Power Cables

• Place the negative test lead to the conductor(s) (one or both conductors depending on the scenario)
• Clamp the positive test lead to the ground wire or a fixed point to ground.
• Select the appropriate test voltage on the insulation resistance meter for most power cables,
• Turn the meter ON and apply the voltage for a minimum of 1 minute

MI Cables

On an MI cable, we are interested in knowing the resistance between the conductors in both the hot and cold leads and the ground sheath of the cable. What we are testing is the resistance of the MgO powder inside the cable along with any contaminants within the MgO (often water or moisture).

• Place the negative test lead to the conductor(s) (one or both conductors depending on the scenario)
• Clamp the positive test lead to the stainless sheath of the MI cable
• Select the appropriate test voltage on the insulation resistance meter
  1. 500VDC for 300V rated MI cable
  2. 1000VDC for 600V rated MI cable
• Turn the meter ON and apply the voltage for a minimum of 1 minute

The higher the resistance value of the meter, the better the insulation. If this value is too low, the cable likely has moisture contaminating the insulation and creating a conductive path from the conductor to the stainless sheath. A common mistake made when meggering MI cables are to attach one lead to each conductor. In general, MI cables are tested at 500VDC or 1000VDC maximum.

SR Cables

• Place the negative test lead to both conductors
• Clamp the positive test lead to the braid SR cable
• Select the appropriate test voltage on the insulation resistance meter. The following tests should be completed
  1. 500VDC
  2. 1000VDC
  3. 2500VDC

Turn the meter ON and apply the voltage for a minimum of 1 minute

Acceptable Values (Pass/Fail)

 Use the table below to verify if the cable has passed or failed IR testing. Table is only applicable to regular system maintenance and cannot be used for new installation of cables and/or commissioning. Ensure testing environment is clean and dry.

Note: Any rapid deflection is considered a fail