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[Miguel Magalhães]

Hi everyone,

Many years ago I had my power distribution box (63 Amp Three Phase) doing fireworks in the middle of an event and it was a scary and stressful situation because where it was plugged, wasn’t easily accessible so it took sometime to shut it off.

I was in shock because I always do my math and never over power my cables or the total capacity of circuit breakers or the main switch which was near maximum that night.

One option for this nightmare could be some loose screws in the circuit breakers due to moving the box around. A coligue said he would every now and then re tighten all the screws in his power boxes. It maked total sense in stuff that is traveling in a van.

And I was questioning myself if makes sense to do that also to the three phase extension cables.

What I don’t understand is why my power distribution box slowly caught fire and no circuit breaker went off. Shouldn’t they shut down with excessive heat?

Anyways, what’s good practice in your opinion?

[Chris Hindle]

Hi everyone,

Many years ago I had my power distribution box (63 Amp Three Phase) doing fireworks in the middle of an event and it was a scary and stressful situation because where it was plugged, wasn’t easily accessible so it took sometime to shut it off.

I was in shock because I always do my math and never over power my cables or the total capacity of circuit breakers or the main switch which was near maximum that night.

One option for this nightmare could be some loose screws in the circuit breakers due to moving the box around. A coligue said he would every now and then re tighten all the screws in his power boxes. It maked total sense in stuff that is traveling in a van.

And I was questioning myself if makes sense to do that also to the three phase extension cables.

What I don’t understand is why my power distribution box slowly caught fire and no circuit breaker went off. Shouldn’t they shut down with excessive heat?

Anyways, what’s good practice in your opinion?

Heat doesn't trip a breaker. Over-current does.
Loose wires create heat, heat is not your friend.

Part of yearly maintenance is opening everything up and tightening screws.
Distro, custom extensions, and every cable that doesn't have a molded plug/socket.
Include the SpeakOns and PowerCons in this procedure.
Open up all the amps, Vacuum all the hidden corners, fans, filters, blow out the gunk that collects on the heat sinks. An old toothbrush is really good at this....
Chris.

[Keith Broughton]

Tightening screws on portable panels and cables is a good idea.

[Brian Jojade]

Properly tightened screws in a proper panel shouldn't require regular tightening.

Now, if you're using a power distro panel that isn't designed for road use (ie, a homemade distro) then it's much more likely that things can shake loose in the truck as they may not be designed to handle such vibrations.

If you are concerned with the screws coming loose, a drop of locktite can go a long way in making sure it never happens to you again.

[Stephen Swaffer]

Properly tightened screws in a proper panel shouldn't require regular tightening.

Now, if you're using a power distro panel that isn't designed for road use (ie, a homemade distro) then it's much more likely that things can shake loose in the truck as they may not be designed to handle such vibrations.

If you are concerned with the screws coming loose, a drop of locktite can go a long way in making sure it never happens to you again.

A couple of comments.  Every lug/breaker etc. has a specific torque spec-hardly anyone actually uses a torque wrench/screwdriver-but if it's critical it would be a good idea.  On connections that are run at/near/over full design load thermal expansion can loosen connections over time as can strands shifting. Keep in mind that a properly sized 75 deg C connection will run close to that temp when fully loaded-and each cycle tends to move material just a little more.

One of the most common issues found when I do infrared thermography predictive maintenance is loose screws-even on stationary equipment.  I usually have no way of knowing if they were properly torqued to start with.  Almost anytime I am working in a breaker panel I will quickly run through to check the tightness of screws-almost always there are a few loose screws usually on heavily loaded circuits.

I would think an off season double check would be best practice, if not essential.

[Tim McCulloch]

I would think an off season double check would be best practice, if not essential.

^^^ THIS.  Our shop used to pick a couple days in Jan or Feb and the tech staff would come in and we'd open everything electrical - quad boxes and AC stingers, L21-30 plugs and connectors, Cam-Loks, Edisons, etc.  Two techs would tag-team the distros.  Everything we could reasonably reach with a screw driver or appropriate wrench was snugged up, visually inspected for heat, arcing, etc and given an Ohmmeter check.  We'd order in BBQ or Chinese food.

These inspections inevitably led to discussions of how/why we'd do things a certain way.  One year these discussions resulted in an overhaul of how we packed cables, what items were put into "kits" for specific gear packages, and other logistics things.  Getting input from multiple workers was most helpful in making for a good day at the gig.

[Ike Zimbel]

A couple of comments.  Every lug/breaker etc. has a specific torque spec-hardly anyone actually uses a torque wrench/screwdriver-but if it's critical it would be a good idea.  On connections that are run at/near/over full design load thermal expansion can loosen connections over time as can strands shifting. Keep in mind that a properly sized 75 deg C connection will run close to that temp when fully loaded-and each cycle tends to move material just a little more.

One of the most common issues found when I do infrared thermography predictive maintenance is loose screws-even on stationary equipment.  I usually have no way of knowing if they were properly torqued to start with.  Almost anytime I am working in a breaker panel I will quickly run through to check the tightness of screws-almost always there are a few loose screws usually on heavily loaded circuits.

I would think an off season double check would be best practice, if not essential.

Yes, definitely, all this gear needs annual maintenance. Of interest: ALL screws heat up to a certain extent when you tighten them up (from friction). Then, they cool down and are not as tight as they were. I find loose hardware in gear that I'm servicing all the time, even brand new gear. It's a good practice to tighten things up, leave it over night and then re-check the next day.

[Daniel Levi]

I know when I do portable appliance testing at work the tightness of screws on rewireable plugs is always checked, checking the plug is part of the test and I would feel it wrong not to make sure the screws were tight.

On consumer units (fuse boxes/distribution boards) over here the screws are supposed to be tightened to the manufacturers torque requirements to retain compliance.

[John Roberts {JR}]

I replaced one old wall outlet in my bedroom that I noticed getting unusually hot. It wasn't caused by a loose screw just resistance between the plug blade and socket. I noticed it when I unplugged a heater line cord and the plug was hot. No bueno...
===
I recall house/trailer fires caused by cheap aluminum wiring that loosens over time due to multiple thermal cycles.

JR 

[Bob Taylor]

Aluminum wire has certain characteristics you have to understand to use it correctly.

Aluminum is a flowing metal, that what us electricians call it. It moves away from pressure. You tighten it up and a year later it is lose again. Also you MUST USE lugs rated for ALR/CU by code, that is suppose to help slow this down, but I don't think it solves it from what I've seen thru my many years doing this.

Aluminum Oxide is also not conductive, Unlike Copper Oxide that at least is a Semi Conductor. That means that as soon as Aluminum is exposed to the air, it oxidizes instantly. This oxide insulation has to be broken to make a connection. When you tighten the lug it grinds thru this oxide and makes a metal to metal, NO Oxygen connection. Also using a No Lox paste that is designed for Aluminum connections, helps to protect the Aluminum from being exposed to the atmosphere and oxidizing.

Side note, when you touch your Voltmeter leads to the Aluminum wire you have seen that sometimes it shows no Voltage? That is that damn Oxide screwing with you. Your leads have sharp points on the end to allow you to press thru the oxide to get a Metal to Metal connection so you can read the Voltage. Use the pointed ends, not the sides or you may get yourself into a Hot circuit even though you thought it was dead.

And I Hate Aluminum wire, but it always gets used when the Budget rules the job instead of quality.

Bob T

[Paul Johnson]

You should have a test regime - especially when 63A can mean considerable heating from arcing. Most modern electricians now use torque screwdrivers to do terminals up to the manufacturers recommended tightness - the old turn till it stops technique being considered old, now.

Distros can also do nasty things if they have current transformers and one of those thing cables comes dislodged - nasty shocks come from CTs! Anything with castors or handles that gets bumped in and out probably needs more than an annual check. The bumping can also upset RCDs making them fail to trip.

[Chris Hindle]

I replaced one old wall outlet in my bedroom that I noticed getting unusually hot. It wasn't caused by a loose screw just resistance between the plug blade and socket. I noticed it when I unplugged a heater line cord and the plug was hot. No bueno...
===
I recall house/trailer fires caused by cheap aluminum wiring that loosens over time due to multiple thermal cycles.

JR

In the past, Aluminum wiring on aluminum-rated outlets was "good". Standard outlets? that was a call to 9-1-1 in the not so distant future.
Is aluminum even used in household wiring these days?
I know it "fell out of practice" in the mid 70's, but it is not outlawed. *I think*
I'll have to have a look at the local renovation super-store and see if they carry the aluminum rated outlets and switches....
Chris.

[John Roberts {JR}]

It seems like fire insurance would be more expensive.

One of the very few things they did right while building my house was using copper wire, they didn't bother with a third ground wire.

JR

[Geoff Doane]

In the past, Aluminum wiring on aluminum-rated outlets was "good". Standard outlets? that was a call to 9-1-1 in the not so distant future.
Is aluminum even used in household wiring these days?

It may not be used for branch circuits, but at least here (Nova Scotia) it's allowed to feed sub-panels.  When I upgraded the electrical in my house a couple years ago, the old main panel became a sub-panel, fed from the new 200A main panel.  The electrician asked if I wanted to save a few bucks and use aluminum instead of copper for that 60A feed (it had been a 125A panel, but most of the heavy loads were moved to the new panel).  I declined, and spent the extra for copper.

Maybe I'm just overly cautious.  After all, the feed in from the street is aluminum, but it's terminated with some serious crimps.  It's also now a shorter run, directly to a pole with the transformer on it, so that's another plus in my book.

As for re-torqueing screw terminals, when I worked in transmitter maintenance at my old day job, it was a once-a-year routine item to re-torque every power terminal in the various transmitters.  We weren't sophisticated enough to actually use a torque driver for the job, but you could often get a fraction of a turn on some connections.  And this was often equipment that had already been in service for 10 or more years.  I recall one connection in an electrical panel that was loose and got hot enough to make the 200A 3-phase breaker trip.  The heat caused the copper wire to lose its temper (and scorch the insulation), so that piece of 250 MCM had to be replaced.  We called in an industrial electrician for that job.

One more thought on this topic:  Bolt-in vs. snap-in breakers in portable panels.  I've always used Square-D QO (rather than QB) panels for my portable stuff, partly because it's cheaper and easier to find, but it also seems that the spring fingers might maintain contact on the bus bars better than screws, which could in theory, loosen up.  Does anybody have have any opinions on this, or better yet, data?

GTD

[John Roberts {JR}]

FWIW there is a lot of aluminum wire up on power poles (with steel core because aluminum is weak).

JR 

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