In the scam, someone calls homeowners or businesses and says their power will be shut off unless they make immediate payment.
The scammers tell the targeted customer the only way to keep the power on is to pay quickly using an untraceable pre-paid debit card, such as the Green Dot or Vanilla Reload Network card, or by making a Western Union money transfer.
Utility officials say they never demand instant payment over the phone and don't require the use of pre-paid debit cards.
Customers scheduled for disconnection due to nonpayment receive sufficient written notice.
NStar and Western Mass. Electric are both divisions on Northeast Utilities.