IRA FLATOW: The holiday season is a time to reunite with family, share the goodwill and show off your house decorating skills, right? You got your Christmas trees, your holiday lights, your animatronic snowman. But there can be dangers.

One of my favorite movies. Remember that scene in National Lampoon’s Christmas Vacation where Clark Griswold plugs in his holiday lights, blows out all the fuses and the cat is vaporized after chewing on the Christmas tree light cord? My next guest is here to keep us grounded. Get it? Safe to make sure your holiday cheer stays bright, but not too bright. Shannon Pruitt is an electrical engineer– yes– and a programming manager at UL, Underwriter’s Laboratories based out of Northbrook, Illinois. Welcome to Science Friday.

SHANNON PRUITT: Hey there, thank you so much for having me on.

IRA FLATOW: As someone who wanted to be an electrical engineer, I’m thanking you for taking time to be with us today.

[LAUGHING]

IRA FLATOW: Some people string dozens of lights together, and then they wrap their entire house in holiday lights. Is there a maximum number of lights that we should put together?

SHANNON PRUITT: Very good question. Ya know, it really all comes down to how your house is wired, how you have wired up the lights that you’ve strung on your house, how you’ve divided those up between the circuits that you have on your house. As long as you are stringing up those lights and not exceeding any ratings on extension cords or receptacles on your house or circuit breakers, et cetera, then more power to you, put up as many lights as you like.

IRA FLATOW: Let me see if anybody wants to call in with a question. 844-724-8255– I’ll give out the number. Do the lights have an expiration date? What’s a good way to check if you need to get a new set of lights?

SHANNON PRUITT: Lights don’t particularly have an expiration date. Although as we know, they can go bad. And most of the time that’s due to kind of the conditions that they’re stored in, how they are stored, how they’re handled and taken care of. So what we like to tell people is that regardless of how old your lights are, whether they’re a year old or 30 years old, that every year when you take them out to decorate your house, you should inspect those lights.

And that boils down to taking the time to run your hand down the wires of the chord to see if you can feel any breaks or cracks in the insulation, looking at the light sockets to make sure that they’re not cracked as well. Those are the types of things that can lead to fires or shock hazards inside the home or outside the home. And we also like to say that if you do find a cracked wire or a cracked bulb or you see a bare copper wire, please, please do not try to fix that yourself by putting some electrical tape around it. It’s much too high of a risk. The light strands are cheaper and cheaper as the years go by, just throw it out and please buy a brand new set rather than trying to fix it yourself.

IRA FLATOW: And then the new ones are LED bulbs. They must be a terrific advantage in terms of power consumption.

SHANNON PRUITT: A huge advantage. Up to 70% savings in the amount of electricity that you will use over the traditional bulb especially depending upon what size of original bulbs that you use to use. Growing up I remember helping to string up the super huge C9 bulbs on the outside of my parents house, and then running around to the side to see the electric meter starting to spin like crazy when you plug them in.

[LAUGHING]

SHANNON PRUITT: But now with the LED lights, and they’re becoming more and more affordable. And many places even offer, you bring in a strand of old lights, and they’ll give you an instant rebate on some of the newer LEDs. Much, much less power consumption over a standard light.

IRA FLATOW: All right, let’s go out to the outside of your house now a little bit and let’s talk about the electricity running around the outside and specifically the kind of sockets that are out there that you need and that kind of extension cord that you might need. What do people do wrong here?

SHANNON PRUITT: So the biggest thing that we find that people love to do wrong is they go to string up the lights, and the end of the light strand just won’t quite reach to the outlet on the side of the house, and so they run in, and they’ll grab their indoor rated extension cord and they will plug it up outside and plug it in, and it works, it lights up the light. But there are huge, huge issues with that. Those chords are not rated for outdoor exposure. Whether the frigid temperatures, the type of climate that you find, especially in the central area of Illinois where we’re at with the cold and snow and things of that nature. So the biggest issue that we find are people will use the wrong types of extension cords when they go to hang outdoor lights.

IRA FLATOW: And then when you say the right kind, you have to have one that can handle all the electricity going through it, right?

SHANNON PRUITT: Yes, and that is another thing that I like to talk with people about, especially as long as you’re using an outdoor rated extension cord. Most of those are pretty well made and pretty highly rated as far as amperage and wattage that they can handle. The issue that we typically find is with indoor decorations, and people go and grab their indoor extension cord. What many don’t realize is the indoor extension cords are not rated to handle the same amount of power that the outlet on your wall is rated for.

And so what happens is they will start connecting things to it and connecting more and more strands of lights and series. And generally what happens this time of year is an extension cord will become overloaded, which leads to the insulation on that cord getting very, very hot and ultimately it will melt causing the conductors on the inside of that cord that short together and then you have lots of sparks and a huge risk of fire at that point. And that can all happen prior to your outlet being overloaded and the breaker or fuse popping or blowing in your home.

IRA FLATOW: That is good to know. We’re talking with Shannon Pruitt of Underwriter’s Laboratory on Science Friday from PRI, Public Radio International. A few listeners are calling in. So let’s go to the phones. Let’s go to Gary in Pittsburgh, Pennsylvania. Hi Gary.

GARY: Hi, how are you?

IRA FLATOW: Hi there, go ahead.

GARY: Yeah, I’m trying to plug in some outside lights into an outside socket, and the lights are coming on or going off depending on where I positioned the plug in the outlet. And typically they’re staying off, and I’m wondering if this is indicating a hazardous condition in my outlet and do I need to replace my outlet?

IRA FLATOW: Good question.

SHANNON PRUITT: Excellent question. So the way I would tell you to troubleshoot that is to get a different set of lights or something else that you can plug into that outlet on the outside of your house and see if whatever that is that you plug in there stays on 100% of the time, regardless of how you position it in the outlet itself. And if it does stay on 100% of the time, then I would say that it is not necessarily the issue with the outlet that you have, but with the strand of lights that you’re actually plugging into it. Very common because those strands of lights you use typically very small gauges of wire that those wires especially around the plug can become brittle over time and will crack. And then depending upon how the wire is slightly positioned here or there, it will make enough contact that the strand will light up, but yet if you move it slightly this way or that, then it loses a connection, and the strand goes out. So my bet would be that it’s more or less the light strand causing the issue versus the outlet. But you never know, it could be either.

IRA FLATOW: But with an outlet and it’s wet outside, don’t you need a ground fault interrupter outlet, one that will detect before you get electrocuted and shut off?

SHANNON PRUITT: Absolutely, absolutely. If anyone who has outlets on the outside of their house per code, per the National Electric Code, should be a ground fault circuit interrupter, GFCI type outlet such that it will detect instantaneous shorts in such a short of the electricity going through you to the ground and stop that from happening prior to a shock hazard occurring.

IRA FLATOW: But I found in checking my own, they can go bad, and still the electricity will run, but the GFI is not tripping, so you need to check on that.

SHANNON PRUITT: They can, yes, they can go bad. And I talked to a lot of kids when I go around to schools doing safety presentations, and I tell them to go have a lot of fun with their parents by going around the inside of their house and pressing all the little buttons that they see on the GFCIs. And this applies to the outlets on the outside of your house as well. They can go bad, and that’s why they have the test buttons on there.

People should take the time to press the button. You should hear a very distinct pop sound, which is the circuit breaking. I also tell people plug something into the outlet, a hairdryer something and turn it on. When you press that test button that appliance or whatever you have plugged in should instantaneously shut off. And if it does not, then that outlet needs to be replaced. And it is common that after years of use that those can go bad and they do need to be tested.

IRA FLATOW: I can testify to that. Some great hints Shannon, thank you for taking time to be with us today.

SHANNON PRUITT: Thank you so much for having me.

IRA FLATOW: You’re welcome. Shanna Pruitt a Program Manager at UL. You know the little UL sign Underwriter’s Laboratory that’s based out of Northbrook, Illinois.

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