Failed electrical cable. Is there a physicist in the house?

[bungeejumper]

Here's a puzzle that's been on my mind for the last few days. I recently overcooked a length of twin-core bell flex that I'd been using to carry a 24 volt AC current at 11 watts. (That's, errr, roughly half an amp isn't it?) But just before the flex died, it gave every appearance of delivering more electrical oomph than usual. Is that possible? (I wouldn't know - I was banned from physics lessons at school after an unfortunate incident with a car battery. )

Okay, the details. The bell flex was being used to extend the low-voltage juice out into the garden, to a Hozelock pond pump (24v AC, consumption 11 watts); the feed was from the (correct) Hozelock plug-in-the-wall transformer in the garden shed. (11 watts output, 15 watts total power consumption, 24 volts AC). I'd have thought that bell flex would have handled that all right, wouldn't you? But as it turned out I was wrong.

So when the pump stopped working, I found that the insulation on the flex had gone very hard ,and the flexible copper braided cores within it seemed to have gone very fragile and brittle. I've never seen that before. But then the hindsight hit me....

For the last couple of days, the fountain jet had been shooting rather higher than usual - so much so that we were losing water! Could there be a connection, and if so, what? Does a nearly-knackered cable deliver more watts, and if so, why?

FWIW, I replaced the bell flex with lighting-grade twin core and earth, and the water flow is now back to normal and everything is fine and dandy. I am therefore disinclined to think that the transformer was to blame. But grateful for any thoughts from the assembled multitude.

TIA

BJ

[Alaric]

Does a nearly-knackered cable deliver more watts, and if so, why?

Less resistance, so more power.

I don't know about the wattage, but don't bells usually run off a single battery, so bell wire is designed for low voltage, low amperage ?

You are familiar with old fashioned electric fires no doubt? These produce heat by running electricity along a wire. Any wiring is apt to do this, if you put more volts and amps through it than it's designed for.

[johnhemming]

t'Internet seems to say that the resistance of copper increases with temperature. Hence I would start on the assumption it was not the cable.

[XFool]

It seems only likely if there had been a short-circuit in the cable or at the pump motor. Also, an electric motor, if stalled or slowed through mechanical resistance, will take a heavy current which could account for the burnt out cable.

I wonder why the water fountain was higher that usual. Was it higher but a weaker spray? Possibly something temporarily obstructing the pump causing a slow down and a heavier current?

[bungeejumper]

I wonder why the water fountain was higher that usual. Was it higher but a weaker spray? Possibly something temporarily obstructing the pump causing a slow down and a heavier current?

No, the water throughput was altogether more forceful than usual, which struck me as odd. It's a good point, though, that an obstruction in the pump might have increased the current. Unless, that is, some sort of integral limiter in the power supply had kicked in to prevent that from happening?

Mysteries, mysteries. Thank you everybody - I think I'll just have to watch and wait to see what happens now? I'm grateful that my outbuildings have built-in circuit-breakers, though.

BJ

[quelquod]

I wonder why the water fountain was higher that usual. Was it higher but a weaker spray? Possibly something temporarily obstructing the pump causing a slow down and a heavier current?

Seems most likely. Is it only a fountain or is there also a pumped output which became blocked?

‘Bell wire’ is usually rated at about 1A though there’s no knowing quite what you’ve got of course. Bells only take current for short periods so they don’t put a lot of strain on the wiring. Perhaps your wire is a bit thinner than normal too and has been running close to its capacity continuously for a lengthy period which would explain the marked overheating. Bell wire won’t be very weatherproof and less so if the insulation deteriorates so air will have reached the conductors corroding them and aggravating the heating. Whatever, the wire won’t have magically increased the power supplied.

[Urbandreamer]

Ignoring for a moment any discussion of the cable selection, it is worth pointing out that our entire view upon the electrical performance during it's end of life is based upon the action of a pump connected to it.

(what is it about LTF and pumps by the way?)

Anyway, a pump is not a pump which is not a pump. There are in fact many different types of pump that often pump different "fluids" and have different processes. For example if you take a positive displacement (ie piston) pump and block or constrict the outlet you will need more energy to attempt to operate the pump. However if this is a centrifugal pump then you will need LESS energy than with a open outlet!

Then there is the question of what is being pumped and from where. Centrifugal pumps are really not very good at sucking. They need some external force to supply the intake (ie you need to submerge them so that air pressure will feed the fluid in).

Finally we are to judge things upon the height a jet reaches, rather than the mass of water pumped. Is it possible that less water was being pumped, but at a heigher velocity? For example because the motor was stopping then starting, accelerating up to 3000 rpm rapidly and hence providing a small slug of warter with a very rapid acceleration (hence more mommentum).

https://www.waterworld.com/municipal/te ... -practices

During the startup of a centrifugal pump, prior to the time that normal flow is reached, these transient conditions can generate heads and input torques (power) that are much higher than design.

So, more Watts, but possibly less Watt-hours. Heigher currents etc.

[XFool]

If a Hozlock EasyClear (model 3000?) it produces a fountain and also pumps water around the pond through a filter.

https://www.hozelock.com/product/easyclear/#

This seems to leave plenty of scope. Probably uses a single motor/pump for both functions? So perhaps blocked filter = higher fountain? Or not!

"Water bypass allows continued UVC and fountain operation if foam is blocked"

[AleisterCrowley]

UV (sunlight) degrades cable insulation, so for outdoor use you need to buy UV resistant cable.
Have you checked the transformer output voltage ? If it's 24VAC out it's prob just a normal transformer rather than a switch mode jobbie, so unlikely to go high when faulty

[MyNameIsUrl]

Suppose there were an intermittent fault in the primary windings of the transformer which shorted out a few turns - the voltage in the secondary windings would go up. Say it went to 30v instead of 24v - the water spray would be higher, but the additional current would fry the bellwire. The intermittent fault has been disturbed as you've rewired, and all is ok for now. If the spray height goes up again, you could confirm this theory with a voltmeter.

(Maybe this is all rubbish. I am not a physicist)

[jfgw]

Pond pumps usually have induction motors. These run at an almost constant speed irrespective of the load. They will not run appreciably faster if the voltage is too high, they will run at about the same speed as normal, take a lot of current and burn out. It is possible for a series-wound motor (a type of motor with brushes) to run faster if the voltage rises or if it is faulty but not an induction motor.

Bell wire is pretty thin but it would have to be exetremely thin not to take half an amp. How long is the wire? It would take quite a few watts to overheat an appreciable length, more than the pump takes. This would result in a voltage drop and, threfore, too low a voltage to the pump.

What model pump do you have?

Julian F. G. W.

[jfgw]

Suppose there were an intermittent fault in the primary windings of the transformer which shorted out a few turns - the voltage in the secondary windings would go up. Say it went to 30v instead of 24v - the water spray would be higher, but the additional current would fry the bellwire. The intermittent fault has been disturbed as you've rewired, and all is ok for now. If the spray height goes up again, you could confirm this theory with a voltmeter.

A shorted winding in a transformer effectively shorts out the whole transformer. It is like shorting out the secondary - the primary will take a lot more current until something (such as a fuse) fails. There would have to be a simultaneous short and open circuit for the secondary voltage to go up.

Julian F. G. W.

[johnhemming]

Perhaps the mains pressure was higher.

[bungeejumper]

Pond pumps usually have induction motors. These run at an almost constant speed irrespective of the load. They will not run appreciably faster if the voltage is too high, they will run at about the same speed as normal, take a lot of current and burn out. It is possible for a series-wound motor (a type of motor with brushes) to run faster if the voltage rises or if it is faulty but not an induction motor.

What model pump do you have?

https://www.keengardener.co.uk/hozelock ... -pump.html. There doesn't seem to be anything about an induction motor, but it certainly kicks up quite a column of water![/quote]

Bell wire is pretty thin but it would have to be exetremely thin not to take half an amp. How long is the wire? It would take quite a few watts to overheat an appreciable length, more than the pump takes. This would result in a voltage drop and, threfore, too low a voltage to the pump.

Certainly agree that bell wire was probably an optimistic choice on my part! No point in my being disappointed (I'm not), but I'm a bit surprised all the same. Should have taken 11 watts, surely? Even a hand lantern often carries more than that. Length of the cable was about eight metres.

Great ideas coming up. Thank you, everyone!

BJ

[AleisterCrowley]

Bog-standard thin hookup wire (7/0.2) is rated at 1.4A

[johnhemming]

Should have taken 11 watts, surely? Even a hand lantern often carries more than that.

It is only the current that matters at voltages the insulation can cope with. The length of the cable should not have any material impact.

[quelquod]

Bog-standard thin hookup wire (7/0.2) is rated at 1.4A

RS Components specs it at 1A, but don’t give their reasoning.

... The length of the cable should not have any material impact.

Not strictly true of course as the wire has resistance. In this case about 1.5 ohms round trip worth about 0.75V (assuming that this is what the ‘bell wire’ is).

[AleisterCrowley]

Bog-standard thin hookup wire (7/0.2) is rated at 1.4A

RS Components specs it at 1A, but don’t give their reasoning.

being a bit cautious? - it's been 1.4A as long as I can remember - used to deal with it all the time !
Just checked Farnell and it's 1.4A on there @70C

[Urbandreamer]

If we are talking electricity, it is worth pointing out that there is a significant difference between a "hand lantern" and a motor.

Your standard bulb is a resistive load.

ie
11W / 24V < 0.5A

while a motor is a inductive or reactive load.

Hence
11W * pf / 24V > 0.5A

Where pf is the "power factor".

I suspect that the result would still be less than 1A while running, but significantly more when the motor starts.

ie 0.5 * 20 = 10A
or 0.5 * 15 = 7.5A
Would be my guess depending upon motor load on start.

Among other things the transformer means that this won't blow a fuse. It will also exist for fractions of a second.

However bell wire was definatly a poor choice.

[jfgw]

Pond pumps usually have induction motors. These run at an almost constant speed irrespective of the load. They will not run appreciably faster if the voltage is too high, they will run at about the same speed as normal, take a lot of current and burn out.

After a bit of research, it seems that pond pumps usually use permanent magnet motors. The same principles apply, however. If the stator is just a mains frequency coil (with no fancy electronics), the speed will be fairly constant.

Julian F. G. W.