iPhone chargers shorting out?

I've read a few posts here about Apple chargers, but my situation is a little different. My husband and I both have the iPhone 6. I had gotten mine on launch day and he got his in February. For about a month now, I'd plug my iPhone in to charge, and it would for a while, then just stop working. The cables were fine, but the chargers themselves wouldn't. It happened 4 times. I did a chat with Apple and they instructed me to go to my local Apple Store, with the chargers and my phone, and get replacements. I did that last evening. I was given a new iPhone 6 and 4 new chargers. All Apple chargers that came with our phones and iPads. Today, my husband goes to charge his iPhone 6, and the same thing happens. The charger just dies. He checked the voltage with all our outlets and everything there is normal. What is causing this????