Author Topic: Crappy under rated Leads.  (Read 644 times)

glort

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Crappy under rated Leads.
« on: March 22, 2018, 11:55:00 AM »

I have been making up a lot of little controller and devices of late to do different things.
 Auto watering system for the aeronautics, made an auto switch for the Christmas lights that turned them on at dusk and off again 6 hours later, temp switch to control the cooling fan on the solar inverters, and significantly, the voltage controller for turning on the hot water heater when the solar is producing power.

All of these things I have been hooking up with some small extension leads I have been buying from our national  hardware chain.  It's cheaper to buy a 3M lead with the plugs on it than just the ends alone and having to stuff round wiring them up and buying cable at well. I cut the leads in half and then have my in and out leads and just wire them up.  Been going though so many of the things I have been buying them 5 at a time and keeping a stock.

I built a box about a month or so back for the hot water controller setup. 
The power goes in to a breaker which a watt meter with inductive pickup monitors. That goes through to the voltage monitor, to a larger mechanical relay, then to a PWM controller with readout, and onto an outlet socket. I didn't use the female end of the lead this time. Because the hot water heater has a 3.8 Kw element and I'm running it for the moment off a 2400W outlet, I avoided the overload by using the PWM which I can see on the meter and have verified with my multimeter to be outputting just under 1000W. I had it at 1500w but the Kettle in the kitchen is on the same circuit and I tripped the breaker a couple of times so backed it down. No worries since.
Takes longer for the water to heat up but no rush and leaves plenty of room on the circuit for other things as well.

The leads I have been using are rated to full outlet current of 2400W.

The other day I was out the back and moved the lead and it felt pretty warm but it was a cosy 38oc day so nothing felt cool anyway.  Checked the current meter and saw the heater was on and then could hear my daughter in the shower so all good.  It rained the other night and wife told me wind was blowing in the rain  under the veranda and was my outlet on the ground OK? It was but I thought I'd unplug it any way just in case it got too damp and tripped the rcd.
I went to unplug the lead going to the box and found it had burned itself into the other plug I was running it into which was a home made one with 2.5MM wire and HD plugs.

I put the voltage relay into manual mode and saw the heater was pulling 980W. Went and got my clamp meter and confirmed that's what the thing was pulling. Wasn't over load it was under rating of the lead itself.

Pretty amazed these leads would heat up as much as they have when the most I have put on them is 1500W.  This bit of lead is about 1.5M so certainly not getting hot through resistance cause by length.  The leads are sold up to 25M on the same ( inferiour) spec wire and clearly rated to 2400W.
I would have thought that something sold by a large chain probably in the thousands a week would be better than that. I can warm these things up way too well just with a 100w load let along a 2400w as they are rated. The fact the things have burned obviously at 1500 is a concern.
My home made lead I did primarily for the welder is clearly fine but these other things.... Yikes.

I went and had a look at some other leads today and although many were marked heavy duty, they are still the same 1.0mm core.
There are some other really expensive ones with 1.5MM lead and still rated at 2400W. I think they might even struggle at the rated load.

Clearly these leads are fine for real light duty applications but can't even hold 2/3rds their rating without being stressed at only a 1.5M length. If I were running the things at 2000W i'd not be surprised at them warming up but 1000? That's bad! If I were running a 25M lead at 1500W and it got warm I'd think probably to be expected,  but as short as I have and still getting hot??

I think the worst lead I have seen so far was on a  Cup water heater element I bought last year.  That was rated at 2000W and the leads would not have done as speaker wire in a crappy factory car sound system. I wouldn't even plug these things in because if that element was even good for 500W, the lead would be putting out more heat than the element would have. Couldn't change the lead as it was molded onto the element so in the bin it went with a suitable feedback to the seller.

The way things are here, it's illegal for me to make up my own leads not being a licenced sparky and not megga-ing them and tagging them to electrical cert.
Fact of the matter is what I have made up is good for twice the rated load of the outlet its plugged for but the legal leads would be an outright danger anywhere near their rating.

Can see myself buying some more 2 core+E round lead and the plugs and making up some leads far superior to anything I can buy off the shelf.



BruceM

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Re: Crappy under rated Leads.
« Reply #1 on: March 22, 2018, 02:35:09 PM »
The issue here is that your PWM pulsed load causing substantial additional inductive heating and a large reactive load heating the wires. This is a well known problem.  It has become the norm to have a high percentage of the total load being non-linear loads, like switching power supplies, and frankly, the power systems weren't designed for that.

Inductance is the property of wires that causes resistance to higher frequencies.  Because the PWM unit is attempting to chop the waveform and introduce rising and falling edges 1000x the line frequency, the effective wire resistance is MUCH higher than the rating- which is for resistive only loads. 

The problem will affect more than just your cord...the wiring feeding that circuit will also be affected.  While switching supplies aren't nearly as bad as your high power PWM unit, they do cause this problem and this has been the cause of many fires.

On the AC input to the PWM unit, adding motor run capacitors to cancel the reactive load may help with supply wire heating by reducing the reactive portion of the load.  With a good true RMS meter, measuring AC volts and Amps while adding capacitance to reduce same is the normal procedure.  I don't know how well that will work with the PWM unit but it's worth a try. 

On the output to water heater side, trying to fix it will cost more than it's worth, I suspect.   

A transformer or autotransformer (Variacs are typically adjustable toroidal autotransformers with no secondary winding and are highly efficient) will reduce the water heater load to your desired 1000W without causing this issue.  If you want to use the PWM unit, check the wire size on that circuit carefully.  I'd want at least 3x the linear rated capacity. 



« Last Edit: March 22, 2018, 05:33:58 PM by BruceM »

basewindow

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Re: Crappy under rated Leads.
« Reply #2 on: March 22, 2018, 09:51:37 PM »
Your leads are probably made in China and relate directly to the power rating on Chinese Inverters. They'd probably work perfectly together.
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BruceM

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Re: Crappy under rated Leads.
« Reply #3 on: March 22, 2018, 10:55:14 PM »
Check the wire size one of the units against a rated 230V current chart and see if you got sold a bogus rated cord.
You mentioned 1mm wire; that's typically rated 9.5 amps for a cord.  At 230V that would be a bit under 2200 watts.  A 1mm cross section area is slightly more so you might see a 2400 watt rating.  This would indicate that the problem is as I previously stated. 

I do agree that all  inverter specs should be viewed with skepticism.  Again, if a big portion of your load is motors, switching power supplies, and other poor PF reactive loads,  the inverter has to supply all that reactive current, so the effective capacity will be dramatically reduced. 

Reactive current DOES drain your batteries just as much as "real" current.  I've confirmed this myself while testing my own inverter design after reading various flavors of malarkey online. When you are off grid on inverter you are well advised to understand your loads, and to correct the PF on all your big loads to minimize real power loss.




« Last Edit: March 22, 2018, 11:08:39 PM by BruceM »

glort

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Re: Crappy under rated Leads.
« Reply #4 on: March 22, 2018, 11:37:52 PM »

Thanks Bruce.
Perhaps the problem is not as much with the Cords as it is the ignorant electrical engineer?

Luckily this is only a temp setup, more to test the Voltage Monitors you put me onto which have worked great.  These things really are the ultimate solution to solar activated water water heating. With a small thermostat switch on the tank, one could kick in the mains when the tank fell to the minimum temp and have a setup that does everything the commercial units for $6-1200 do for about $50.

I'll move the hot water circuit in the fuse box from off peak to the normal supply and run the heater at full load as normal.

I did notice the meter was showing too many amps for the volts and watts but put it down to an inaccurate meter.  Learn something every day!

The cords have a 1mm core cross section but I think are still a barely there soloution.  As I said, most cords available are only 1mm with some real expensive ones being 1.5mm.  They are far cheaper to buy the round 3 core cable and ends make them up yourself in 2.5MM. 

I was running a test setup a while back using panels to charge some batteries and then running an inverter and a PWM to control the output of an Urn.
Didn't seem to have any problem with that although the urn was half the power of the water heater and I was running it at a lot less output, about 500W max.

Thanks again Bruce.

BruceM

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Re: Crappy under rated Leads.
« Reply #5 on: March 23, 2018, 01:06:08 AM »
You're most welcome. 



M61hops

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Re: Crappy under rated Leads.
« Reply #6 on: April 08, 2018, 06:06:39 AM »
Also, the same size wire can carry more or less current depending how many strands it has.  I've noticed that cheaper wire has fewer but larger strands for the same gauge.  And some brands of wire work harden and break much more easily than others.  It may be that some wire is less pure copper than other brands.  Good info about the PWM controllers.
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