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Safety Issue - Moulded 13A Plugs


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I wonder whether there may have been some corrosion or surface oxidation on the fuse clip that would have created hot-spots where the current got through.

 

Two instances described in the thread - both in out-buildings - could damp have got in?

Edited by sharris
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I've had to replace the 30 amp plug on our motorhome a couple of times as I saw that the rubber around one of the prongs was melting. (this is 120V)  Our dealer suggested that it might be under-voltage in the campground. I wonder if it might be a few too many heaters inside the RV.

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I've had to replace the 30 amp plug on our motorhome a couple of times as I saw that the rubber around one of the prongs was melting. (this is 120V)  Our dealer suggested that it might be under-voltage in the campground. I wonder if it might be a few too many heaters inside the RV.

More than likely it happened when you were plugged into an outlet that was making poor contact with one of the prongs on your plug. If you were pulling 30 amps and the contact resistance was 0.1 ohms the power dissipated at the plug prong would be 90 watts.

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I've had to replace the 30 amp plug on our motorhome a couple of times as I saw that the rubber around one of the prongs was melting. (this is 120V)  Our dealer suggested that it might be under-voltage in the campground. I wonder if it might be a few too many heaters inside the RV.

 

Shows what your dealer knows :)

 

Simple heaters are resistive and obey ohms law. Lower voltage would result in lower current flowing.

 

I agree with others that it was most likely poor contact or wiring.

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An interesting and slightly worrying topic as i have experienced hot plugs as well

 

In my case the power supply to the outbuilding is about 20 metres of half inch? diameter armoured cable, partly buried and partly attached to a wall. there are consumer units at each end. I have an old 2Kw dimplex fan heater which I use for heating. When used at 2Kw the plug gets hot. At 1Kw the plug gets warm. After reading this topic I wondered if the problem was the socket mounted thermostat so I switched on the heater this morning using another socket for 1 hour at 2Kw HOT. So it is not the thermostat. I have a new dimplex fan heater used in the conservatory so I tried that for an hour at 2Kw and the plug got hot but not as hot as the old heater. A test at 1Kw had the plug getting warm.

 

As a hoped for fix I intend to use the new dimplex at 1Kw with frost protection and remove the socket thermostat. however it would be good to know what the problem is. Could it be that the wiring can't handle 2Kw? It was professionally installed. I have searched on line but the information just confuses me

 

The power consumption out there could now be a 1Kw heater plus a 300 Watt dehumidifier(occassionaly) and 2 long flourescent tubes and 4x25 watt daylight energy saving bulbs. No power for the layout as it is radio control battery power.
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An interesting and slightly worrying topic as i have experienced hot plugs as well

 

In my case the power supply to the outbuilding is about 20 metres of half inch? diameter armoured cable, partly buried and partly attached to a wall. there are consumer units at each end. I have an old 2Kw dimplex fan heater which I use for heating. When used at 2Kw the plug gets hot. At 1Kw the plug gets warm. After reading this topic I wondered if the problem was the socket mounted thermostat so I switched on the heater this morning using another socket for 1 hour at 2Kw HOT. So it is not the thermostat. I have a new dimplex fan heater used in the conservatory so I tried that for an hour at 2Kw and the plug got hot but not as hot as the old heater. A test at 1Kw had the plug getting warm.

 

As a hoped for fix I intend to use the new dimplex at 1Kw with frost protection and remove the socket thermostat. however it would be good to know what the problem is. Could it be that the wiring can't handle 2Kw? It was professionally installed. I have searched on line but the information just confuses me

 

The power consumption out there could now be a 1Kw heater plus a 300 Watt dehumidifier(occassionaly) and 2 long flourescent tubes and 4x25 watt daylight energy saving bulbs. No power for the layout as it is radio control battery power.

The cable has nothing to do with it. The heat is being produced at the plug or socket because there is more resistance there than there should be. Note that a high resistance connection will end up heating both the plug and socket which accelerates oxidation which increases the resistance etc etc.

 

When were the outlets installed? Are they in humid locations?

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Shows what your dealer knows :)

 

Simple heaters are resistive and obey ohms law. Lower voltage would result in lower current flowing.

 

I agree with others that it was most likely poor contact or wiring.

Erm you might like to rethink this post.

 

Ohms law states that if the voltage goes down then the current must go up if the resistance stays the same.

 

Similarly if the voltage goes down then the current must go up to maintain the same amount of power.

 

This is why conductor rail electrification is so inefficient - the low voltage means high currents and extra meaty connections are needed.

 

If your RV heater is trying to consume XX Watts of power and the campsite voltage is low (120V vice 240V) , then more current will be pulled which could exceed the design limits of the plug - unless the RV features a power monitoring system that puts an absolute limit on the current drawn at any voltage.

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Phil

 

Its far, far more likely that a simple heater is a constant resistance device, than that it is a constant power device.

 

I = V/R, so with R constant, the lower the voltage, the lower the current.

 

Kevin

Edited by Nearholmer
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Erm you might like to rethink this post.

Ohms law states that if the voltage goes down then the current must go up if the resistance stays the same.

Similarly if the voltage goes down then the current must go up to maintain the same amount of power.

This is why conductor rail electrification is so inefficient - the low voltage means high currents and extra meaty connections are needed.

If your RV heater is trying to consume XX Watts of power and the campsite voltage is low (120V vice 240V) , then more current will be pulled which could exceed the design limits of the plug - unless the RV features a power monitoring system that puts an absolute limit on the current drawn at any voltage.

NO

 

V=RI lets say it’s a 10 ohm element , at 120 ( lets ignore rms versus peak ) it’s 120/10= 12 amps , voltage drops to 100 , it’s 100/10 = 10 amps , at 120 V it’s a 1440 watt Heater , at 100volts it’s a 1000 watt heater

 

A resistance heating element is not a constant power device

 

Dave

Edited by Junctionmad
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The cable has nothing to do with it. The heat is being produced at the plug or socket because there is more resistance there than there should be. Note that a high resistance connection will end up heating both the plug and socket which accelerates oxidation which increases the resistance etc etc.

 

When were the outlets installed? Are they in humid locations?

 

8 years ago and yes, humidity up to 85% which is why I recently acquired a dehumidifier. humidity now 50%. The sockets are 2 double boxes screwed to a wooden back panel

Edited by davetheroad
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If there is resistance (there’s always resistance, I mean appreciable resistance, unacceptable resistance, let’s not get too picky) between the pins of the plug and the springy fingers of the socket, a few insertions & withdrawals will sort it out, unless the spring fingers are actually knackered. You can also clean the pins of the plug. Don’t try to clean the socket, unless you’re sure it’s not live!

 

In the OP, it seems likely that one or more of the connections between the appliance flex and the pins within the overmoulded plug were unacceptably poor, and these would be the source of a high resistance element within the circuit. Probably a bad crimp or a poor riveted joint.

 

Doesn’t need to be much - as pointed out in an earlier post, the heating is I^2 x R, so a resistance of 0.1 ohm at 13A would give you some 17W. (The current & heating effect would actually be slightly lower, given the marginal extra resistance - 0.5%) Given that this heat source is enclosed within a chunk of plastic, not the greatest thermal conductor, it’s pretty obvious that it’s going to get pretty hot, pretty quickly. (If in doubt, hold a lit car brake light bulb, 21W, it’ll burn your fingers quite quickly)

 

Add the fact that the resistance increases as the temperature increases, and the clamp force of any rivet or crimp is probably going to reduce as the rivet expands due to the heat, and you’re into a vicious circle. It gets warm, then it gets hot, then it gets hotter, and it continues until it fails or it gets turned off.

 

And I don’t think I’m exaggerating to say that pretty much every electrical fire is pretty much due to poor contact, getting worse as it gets hotter. Some actually arc, due to a combination of vibration, and looseness. That sets fire to anything flammable in close proximity very quickly.

 

So if you’re doing a bit of wiring, make your connections well, and make sure you do the screws up tight!

 

Happy 2019

Simon

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Erm you might like to rethink this post.

Ohms law states that if the voltage goes down then the current must go up if the resistance stays the same.

Similarly if the voltage goes down then the current must go up to maintain the same amount of power.

This is why conductor rail electrification is so inefficient - the low voltage means high currents and extra meaty connections are needed.

If your RV heater is trying to consume XX Watts of power and the campsite voltage is low (120V vice 240V) , then more current will be pulled which could exceed the design limits of the plug - unless the RV features a power monitoring system that puts an absolute limit on the current drawn at any voltage.

You'd only really get that effect if there was an ac-ac converter in the line to regulate the internal supply of the RV to a constant RMS voltage.

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8 years ago and yes, humidity up to 85% which is why I recently acquired a dehumidifier. humidity now 50%. The sockets are 2 double boxes screwed to a wooden back panel

 

It's hard to be sure without looking at subjects, but I suspect oxidation has a lot to do with it. It might be best to simply have the outlets replaced and have the electrician install new plugs on the cables. I'd be surprised if that does not solve the problem (although I am surprised quite often).

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NO

 

V=RI lets say it’s a 10 ohm element , at 120 ( lets ignore rms versus peak ) it’s 120/10= 12 amps , voltage drops to 100 , it’s 100/10 = 10 amps , at 120 V it’s a 1440 watt Heater , at 100volts it’s a 1000 watt heater

 

A resistance heating element is not a constant power device

 

Dave

 

 

 

A resistance heating element IS a constant power device - its is fundamentally just a coil of wire and unless you introduce other electrical components to regulate it (i.e. a thermostat which alters the overall resistance and regulates current flow) or change its conductive proprieties by altering its molecular makeup (e.g. heating / cooling it) that wire will always pass a certain unchanging electrical current.

 

If the current is too large the wire gets hot and melts - it doesn't magically turn round and somehow start passing less current because its getting warm (if anything it will allow an even grater current to flow leading to thermal runaway). This is basic physics and can easily be demonstrated in in any science lab.

 

As such if it is calculated to require 10 amps at 240V then a drop in voltage WILL cause it to try and pull more Amps to re-balance Ohms law. Its basic text book physics.

 

If on the other hand you introduce extra electrical components which 'fix' the current draw at 10A then a lower voltage will naturally result in a lower power output so as to maintain the Power = Volts x Current equation in the way you quote. However should those 'fixing' components fail then the current drawn may well increase beyond the design perimeters.

 

 

Note:- I have no knowledge of RVs nor the types of heater that may or may not be fitted.

Edited by phil-b259
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A resistance heating element IS a constant power device - its a coil of wire and unless you introduce other electrical components to regulate it (i.e. a thermostat which alters the resistance of the element and regulates current flow) or change its conductive proprieties by altering its molecular makeup (e.g. heating / cooling it) that wire will always pass a certain unchanging electrical current.

 

If the current is too large the wire gets hot and melts - it doesn't magically turn round and somehow start passing less current because its getting warm (if anything t will allow an even grater current to flow leading to thermal runaway). This is basic physics and can easily be demonstrated in in any science lab.

 

As such if it is calculated to require 10 amps at 240V then a drop in voltage WILL cause it to try and pull more Amps to re-balance Ohms law. Its basic text book physics.

 

If on the other hand you introduce extra electrical components which 'fix' the current draw at 10A then a lower voltage will naturally result in a lower power output so as to maintain the Power = Volts x Current equation in the way you quote. However should those 'fixing' components fail then the current drawn may well increase beyond the design perimeters of the plug.

 

 

Note:- I have no knowledge of RVs nor the types of heater that may or may not be fitted.

 

Phil,

 

A heating element is not a constant power device. It's a resistor made of a material that maintains (approximately) constant resistance over a wide range of temperatures. It is definitely NOT a constant power device, and whoever told you that was talking a complete load of sh.....

 

Andy

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Phil

 

I’m sorry to say this and I’m sorry to be blunt, but you’re talking twaddle.

It’s important, because people may remember what you’ve told them, and it’s wrong.

You know, like Donald Trump.

 

It’s a resistor, pure & simple and CANNOT pull more amps than the very stable voltage source supplies.

 

It’s not the current that’s fixed, it’s the voltage. (More or less). Actually, it’s very, very stable.

 

And to pick up on another point, ohmic tesistances actually do change with temperature. See here, for example.

 

https://www.allaboutcircuits.com/textbook/direct-current/chpt-12/temperature-coefficient-resistance/

 

So please verify your comments before posting.

Happy New Year

Simon

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Phil,

 

A heating element is not a constant power device. It's a resistor made of a material that maintains (approximately) constant resistance over a wide range of temperatures. It is definitely NOT a constant power device, and whoever told you that was talking a complete load of sh.....

 

Andy

 

If something has a resistance that doesn't change and it is supplied with an unchanging voltage and an unchanging current then it uses a fixed and constant number of Watts (Power).

 

The important point is that the adjustment / control is performed by an outside actor as it were (unlike say a bi-metalic strip which automatically regulates itself by making and breaking electrical continuity by itself), the element itself regulates noting.

 

A heating element (however well it responds to heat) is still, as far as I am aware at its core, nothing more than a carefully selected coil of wire.

 

It therefore strikes me that the heating element itself must be a 'constant power device' when it is supplied with power of a specified voltage and current.

 

Obviously the heater containing said element is not a 'constant power device due to the 'processing' circuity inside that regulates the voltage and current supplied to the element as determined by the user.

 

However if the heater is rated at a maximum of 50 Watts, and the user keeps it set on 'max' then the heating element will constantly try and use 50W of power and therefore feed the element with a certain voltage and current. Should the voltage drop and the control circuitry for the heater be defective then it may(note not 'will') be possible for grater currents to be drawn from the supply to compensate for the lower voltage and maintain the output.

 

The exact voltage and current which the element may be subjected to is a function of other separate components within the heater.

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Phil

 

ITS A PASSIVE RESISTOR, current is equal to voltage over resistance. Power is equal to current squared times resistance.

 

It’s nothing to do with “external components”. It can’t output 50W if it doesn’t get (at least) 50W input. You’re on the wrong track here. Give up.

 

 

Simon

Edited by Simond
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Mostly the external components in a heater are (1) switches to switch in or out a particular number of elements and (2) a simple thermostat (e.g. A bimetallic strip) that switches on the heater elements when it's cold and switches them off again when it's warm enough.

 

To have an external device that could make more current flow through the element when the supply voltage drops would imbue it with magical powers that let it change the physical properties of the element.

 

As Scotty might say 'Ye cannae break the laws of physics"

 

Technically a switched mode power supply could be used to provide a constant voltage and hence power to an element for a range of supply voltages, but apart from a few very technical applications nobody needs such tight regulation of a heater.

Edited by sharris
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Phil

 

I’m sorry to say this and I’m sorry to be blunt, but you’re talking twaddle.

It’s important, because people may remember what you’ve told them, and it’s wrong.

You know, like Donald Trump.

 

It’s a resistor, pure & simple and CANNOT pull more amps than the very stable voltage source supplies.

 

It’s not the current that’s fixed, it’s the voltage. (More or less). Actually, it’s very, very stable.

 

And to pick up on another point, ohmic tesistances actually do change with temperature. See here, for example.

 

https://www.allaboutcircuits.com/textbook/direct-current/chpt-12/temperature-coefficient-resistance/

 

So please verify your comments before posting.

Happy New Year

Simon

 

Where did I mention a stable voltage source?

 

I am talking about concepts - as in the "give me a lever large enough and I will move the world" type stuff.

 

Obviously a power supply will have limits to what it can supply - but its specifics will depend on its application.

 

Typically laboratory style power supplies can be set to limit current and voltage drawn (i.e. max power out) - and as you say if current limiting is in place then too higher current draw simply results in a progressively lower voltage (I used these many times while at college).

 

A 3rd rail traction supply will have no trouble passing through a human body without tripping the circuit breakers - but a metal scaffold pole (or even a tin can) is enough to trip them. Why? - well thats because the perimeters of the system are such that a human body is usually insufficiently conductive to pass enough current to exceed the maximum the supply can provide, but a nice low resistance piece of metal on the other hand (or simply too many trains in section) causes an immediate trip as the supply finds it is being asked to provide more than it can supply so the breakers trip.

 

In an RV or campsite I would imagine that power supplies are in essence much like those seen in the lab where excess current draw simply results in a low voltage and the power being restricted thus limiting safety issues.

 

However these can occasionally go faulty (as can voltage / current regulators used by the National Grid) with sometimes undesirable consequences - which sort of brings us back to the plug melting.

 

It could be a defect in manufacture (a high resistance contact and arcing) of the plug OR it could be a defective appliance where it has become faulty and is trying to pull that little bit more current because the voltage is low. The most likely candidate is the former, but unless you know the specifics of how the power supply side of the device is handled (i.e. the manufacturer includes under voltage protection, etc) then the latter must remain a theoretical possibility.

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Erm you might like to rethink this post.

 

Ohms law states that if the voltage goes down then the current must go up if the resistance stays the same.

 

Similarly if the voltage goes down then the current must go up to maintain the same amount of power.

 

This is why conductor rail electrification is so inefficient - the low voltage means high currents and extra meaty connections are needed.

 

If your RV heater is trying to consume XX Watts of power and the campsite voltage is low (120V vice 240V) , then more current will be pulled which could exceed the design limits of the plug - unless the RV features a power monitoring system that puts an absolute limit on the current drawn at any voltage.

 

 

 

In an RV or campsite I would imagine that power supplies are in essence much like those seen in the lab where excess current draw simply results in a low voltage and the power being restricted thus limiting safety issues.

 

 

If a RV or campsite tells you this sort of stuff, then what they are doing is saying 'Oh no, there is nothing wrong with the power supply here"!

 

Of course there never is a problem, that needs fixing! It's an expense after all.

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Phil

 

ITS A PASSIVE RESISTOR, current is equal to voltage over resistance. Power is equal to current squared times resistance.

 

It’s nothing to do with “external components”. It can’t output 50W if it doesn’t get (at least) 50W input. You’re on the wrong track here. Give up.

 

 

Simon

 

Personally I like to know how, and where I am going wrong thank you!

 

I may not know anything about RVs or their on board electrical arrangements but I did study electrical engineering so am not exactly unfamiliar with electrical theory.

 

As it happens I don't see anything I have said which disputes the Power = (I squared) x R equation, nor Power = (V Squared) / R for that matter ** and would invite you to show me where I did.

 

A 50W heater running a full power must manifestly consume the highest current and be supplied with the greatest voltage to do so if the equation P=IV holds true. If you run that 50W heater on half power then either the current and / or the voltage to which the heating element is subjected to must have fallen.

 

Equally its entirely possible to design a 50W heather which has a failure mode where it will try and output more than that - for example a 100W heater that is 'restricted' (much like a car may be speed restricted) to not output more than 50W* Granted such a heater might not exist thanks to laws and regulations - but conceptually it could, which could in turn lead to a melted plug. Like I said, I know nothing about RVs and the relevant standards so I accept this may not be something that could occur in practice.

 

 

** Of course if we are talking about an AC supply then resistance becomes a bit of a red herring as we also may need to consider Reactance (as Inductance and capacitance start to come into play)

 

 

* This technique is actually very common in some consumer goods - make everything the same in terms of hardware as its means only one stet of designs and tooling but install a more rubbish version of the software for the cheaper models

Edited by phil-b259
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