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Maximum sustained current perrmitted from 13 amp plug ?
broadgage 70728247
336 Posts
Question
This sounds an overly simplistic question, and the obvious answer is of course 13 amps. There is a clue in the name you know.

However in the case of a simple resistive load, the current will increase at a higher supply voltage. So at what voltage is the current measured for approval purposes ?
At 230 volts, the nominal or declared voltage ?
At 240 volts, the average voltage actually supplied most of the time in most places ?
Or at 253 volts, the maximum permitted. Or even at 256 volts the maximum achieved in an outbuilding with a bit of voltage rise due to grid tied PV on the roof.

The question has arisen due to a number of brand new "fast boiling" domestic electric kettles of reputable make failing a third party  PAT test due to "excessive current"

I repeated the test with my PAT tester and achieved a similar result, FAIL  in the premises in which they are to be used, but "pass" if tested elsewhere.
Tests with a variac and calibrated ammeter showed that a cold kettle on a 250 volt supply uses nearly 14 amps. just under 13 amps at 230 volts, both figures declining a little as the water starts to heat.
Had I done the original testing, I would have been inclined to use a little common sense and discretion and to pass an appliance that uses less than 10% overcurrent for a few minutes.
I would not pass an appliance such as a space heater that used even a slight overcurrent for hours at a time.

Wondered what others think.

And whilst we are on the subject, what about large portable air conditioners ? Available to hire fitted with 13 amp plugs, they appear to be a constant wattage load and use nearly 15 amps if the supply voltage is very low, and almost exactly 13 amps at 240 volts.
24 Replies
Sparkingchip 72796851
2474 Posts
Why was the current drawn being measured?
Sparkingchip 72796851
2474 Posts
Why were brand new kettles being PAT tested?
Chris Pearson 11001208764
1452 Posts
BS 1363 Section 1 Scope: "The plugs and socket-outlets are suitable for the connection of portable appliances, sound-vision equipment, luminaires, etc., in a.c. circuits only, operating at voltages not exceeding 250 V at 50 Hz."

Section 7.6 covers markings, which give the rating as 13 A and 250 V. Have a look on the underside of a plug!

Section 16 Temperature rise says: "The tests shall be carried out at rated voltage."

With an allowance of 4 hours to go from 20 deg C to 70 deg C at 14 A, I don't think that a short overload will do any harm.

 
broadgage 70728247
336 Posts
Sparkingchip:
Why was the current drawn being measured?

Because some automatic PAT testers do this as part of the automatic test sequence, more than 13 amps is a fail. Measured at whatever the mains voltage happens to be at the time of the test.
So an appliance can fail one day with the mains at 250 volts, but pass the next day when the mains voltage is 225 volts.

 

broadgage 70728247
336 Posts
Sparkingchip:
Why were brand new kettles being PAT tested?

Because that is the policy of the customer, and IMHO actually a sensible policy. Several failures have been found in brand new items.

AJJewsbury 77361768
1611 Posts
Several failures have been found in brand new items.
I'd agree with that - in fact in the days when I did a little portable appliance testing the only failures I ever detected during testing were down to manufacturing errors (mainly L-N reversal in moulded leads). There was some damage that occurred in-service, but that was always detected by visual before it got as far as a test meter.
   - Andy.
Sparkingchip 72796851
2474 Posts
I presume they are rated at 3kW.

I have two PAT testers, one mains which will do a leakage test with the appliance under load and one which is battery operated which won’t and has a substitute leakage test.

So it would also depend on which tester I choose to use and which test I choose to run. The testing regime that reveals this “fail” goes above and beyond what is required for a kettle and in this particular situation is being influenced by external factors.

I cannot imagine I would have run a test to measure the current drawn by a kettle in the first place and would not be raising any concerns.

 Andy B
Sparkingchip 72796851
2474 Posts
I thought it would not be a bad idea to turn my PAT testers on, not having done so since March, so I thought I would try the test on our kettle.

Bit of an issue,  the kettle needs to be cold, however I had been asleep so it was. The kettle is rated at 2520-3000 watts, the voltage is hovering around 245 volt.

It is not a test I would normally run for a kettle. Maybe a bit of over zealous testing raising unnecessary concerns?

Sparkingchip 72796851
2474 Posts
That was the current drawn,  the leakage is 0.22 mA.

 
mapj1 80733779
2070 Posts
Good, or I'd have been suggesting you should be checking for an N-E reversal at the plug, a mistake that RCDs have almost eliminated.
Actually I have a feeling that the leakage to earth figures are also better on modern kit, maybe another effect of RCDs and returns from irate customer is that manufacturers are a bit more careful with the moisture control and sealing of mineral insulated elements in kettles, cookers etc..
On the OP, given that the flex on most kettles is clearly only rated for intermittent operation, I imagine we can extend the same latitude to the plug, though how much I'm not sure - if it says 13.5A well all is OK, but if it was nearer 15 probably not, I can see the concern.
Somewhere I have a photo of a real fast kettle we used on field trials where inclement weather and poor shelter meant the normal arrangement took far too long, basically an ordinary domestic model, but wired between 2 phases on a genset to get 400V instead of 230. Very fast but no issues with the 13A fuse or the plug during the time it took to boil. It lasted the duration of the trials and was very popular.
I'm not recommending that of course, and it hinges on the cold weather, but it does suggest that there is scope for allowing a higher 'short duration' rating.  The corollary is that a 13A plug with a long duration load, like an immersion heater, especially in a place that is already warm and badly ventilated, like an airing cupboard,  can suffer from overheating with a load that is less than 13A, so for  a long duration load, the situation is very different. A similar long overload situation may welll apply to the higher power 13A car  chargers, for which improved 13A sockets and plugs are supposed to exist, presumably with more metal and less plastic, though I am not seeing much evidence of them in use.

 
Sparkingchip 72796851
2474 Posts
The tester alternates and displays all the test results in sequence.

One thought is that the Coronavirus lockdown may affect the current drawn, if this is a hotel or similar premises and is currently closed the voltage will be higher than under normal circumstances.

Andy B
AJJewsbury 77361768
1611 Posts
My guess would be that current rating, rather like voltages, are nominal rather than precise actual. A certain margin is expected.

As you say, the voltage may be increased by 10% or more over 230V - and would still be considered 230V nominal.

Similarly there are margins built into overcurrent protective devices - e.g. an MCB shouldn't trip when carrying current 13% above its rating (at least not for conventional time - usually an hour), Likewise cables would be expected to carry those sorts of currents above their ratings for such periods without significant damage (otherwise our whole approach to overload protection - In ≤ Iz falls apart). So it would be logical for the same to apply to accessories such as plugs and sockets (unless their product standards say otherwise).

So I'd suggest that in general up to a 10% or even 13% excess actual current, due to voltage variation, provided it wasn't expected to persist for more than say an hour, should be considered acceptable.
    -  Andy.
Chris Pearson 11001208764
1452 Posts
AJJewsbury:
Similarly there are margins built into overcurrent protective devices - e.g. an MCB shouldn't trip when carrying current 13% above its rating (at least not for conventional time - usually an hour), Likewise cables would be expected to carry those sorts of currents above their ratings for such periods without significant damage (otherwise our whole approach to overload protection - In ≤ Iz falls apart). So it would be logical for the same to apply to accessories such as plugs and sockets (unless their product standards say otherwise).

On that basis, we would ignore BS1363 and go by the characteristics of BS1362 fuses. If it doesn't blow the fuse, it must be safe! 🤨

Incidentally, I have (moulded on) 13 A plugs which are rated at 3 A or 5 A.

I don't think that the test which was carried out is in any way a PAT fail. It is ridiculous to worry that a kettle takes slightly more than 13A at something above nominal voltage, and if the volts were low at 208V a 3kW appliance could quite reasonably take about 14.5A. A 13A plug will carry about 20A for short periods, and this does not cause serious overheating, because of the usual reason forgotten by electricians of thermal inertia and thermal capacity. These items will have passed the manufacturers test, be CE marked etc, and if you want to complain it would be very difficult as you would be claiming the CE mark was invalid, probably from a British manufacturer (wherever it was made) for a British brand. I suggest your PAT contractor is an Id**t and requires re-education and should not be employed again. As the PAT tester does not know what kind of appliance is connected, how does it set the maximum current anyway? It is normally a user setting, so perhaps it is incorrectly set at 13A -+ no tolerance at all as it is accurate to 0.000001%? The whole basis of this current test is nonsense.

For the other comments BG, you are making the same mistake yourself. A heater which takes slightly over 13A (say 14A) is in no way dangerous to plug or anything else. The whole 13A fuse system uses a 13A fuse to prevent overload, The fuse blows at about 17A after about an hour, and this is the safety valve for everything. If I wanted to make it fuse quickly at about 14A, I would have to use a fuse with a continuous rating of about 9A, which you will notice we do not. Why is it that almost no one here seems to understand temperatures, overload currents, fuse characteristics, and thermal capacities here, they are an important fundamental piece of electrical engineering!

Items with a negative input impedance (switch mode power supplies) take more current at lower voltages, but again are adequately protected by fuses of 13A for appliances. This may be plug in induction hobs, chargers, vfd motors etc. Forget the working current, 3 kW maximum is the expected power, a much better indicator of proper operation than the current. Remember the test instrument accuracy (probably no better than 3%) and probably more like 5 or 10% for indicated power, and remember the power factor may not be 1.
ebee 81966746
691 Posts
Mapj1,
ref " N-E reversal at the plug, a mistake that RCDs have almost eliminated."
Could you elaborate on that please? I`m findinding difficulty grasping the reason.
Thanks
mapj1 80733779
2070 Posts
In the days when VOELCBs were common, and most non TT supplies had nothing at all, we had a shiny new scout HQ with a 30mA RCD for the lot.
This was used for leader training, events so on a weekend course,  adults brought in their own shavers radios, rechargeable torches and all sorts.
At that time a lot of kit was in circulation with the older red black green mains leads and people were far more in the habit of swapping plugs from round to square and vice versa,  most did this very well, but a few folk seemed to do their plug fitting in the half light using a nail file and bread knife or something.

On more than one occasion, someone brought something in with the plug mis-wired with an  earth neutral swap, that 'worked OK at home'  that because the black wire was under the earth pin,. perhaps unsurprisingly disappointment when it all went dark, and there was denial from everyone that it could possibly be something that someone had brought in. Funniest like that was a red-green reversal, so the case was live and the load live to earth.
If it really had worked at home, then there must have been further faults in his  home.
 
AJJewsbury 77361768
1611 Posts
ebee:
Mapj1,
ref " N-E reversal at the plug, a mistake that RCDs have almost eliminated."
Could you elaborate on that please? I`m findinding difficulty grasping the reason.
Thanks

Because the load would then be connected L-PE rather than L-N so the RCD would see all the normal load as leakage and trip (unless it was a particularly small load - well below 6.9W).
    - Andy.

ebee 81966746
691 Posts
Hah. Now I feel stupid. I was thinking of a L N reversal. That`ll teach me to read properly. I`m of Red, Black, Green age meeself
GeorgeCooke 11001210072
42 Posts
Many years ago the Post office downrated all 13A plugs used on its property to 8A as they believed that was all they were good for.
broadgage 70728247
336 Posts
Some interesting points made, thanks for the replies.
Had I tested the new kettles I would have  been inclined to pass them in view of the very short term use.

I can not however agree with those who state that a space heater drawing 14 amps is OK. I have seen far too many 13 amp plugs that have failed under very modest overload, or even long term full load. Under modest overload the plug or the socket will often fail before the fuse operates.

It would appear that most appliance manufacturers take a similar view, as most new appliances are limited to about 10 amps, possibly due to warranty claims or adverse publicity about melted plugs.
3Kw heaters are now hard to find as are 3Kw tumble dryers. 
Chris Pearson 11001208764
1452 Posts
broadgage:
It would appear that most appliance manufacturers take a similar view, as most new appliances are limited to about 10 amps, possibly due to warranty claims or adverse publicity about melted plugs.
3Kw heaters are now hard to find as are 3Kw tumble dryers. 

I think that it is more likely to be due to energy saving. IIRC, vacuum cleaners now have a statutory (EU) maximum power.

Mind you, I was thinking about this when I was putting a plug on this afternoon. Those diddy little terminal screws seem very insubstantial.

I was not suggesting that one should operate the plug in any other way than its design. However, it is designed to work with a nominal 3kW load, such as a fire or kettle, and tested for said load. You will notice the use of the word "nominal". The fusing is intended to protect against loads which are faulty in some way and take significantly more current than the intended rating. That is the entire design criterion for electrical installations, and everyone ought to understand that, although some posts elsewhere clearly don't! We never expect a circuit to disconnect at its fuse or MCB rating, and this is expected from the usual time-current characteristics of the CPD. In the case of a 13A plug exactly the same applies, a small overcurrent will never disconnect via the fuse and is not a problem, particularly with a kettle which is never used continuously, and in fact cannot be by design. This means that the suggestion that these appliances are in any way faulty, or worse dangerous, is patently false. The IET code of practice does not mention the current as a test, or give any limit on the exact rating, it is not a PAT issue. Why is this not understood Broadgauge?

I have over many years spent an inordinate amount of time explaining that cables do not immediately melt with overloads, that ring circuits never catch fire (unless some connection is loose!), that a cable running with a 50% on 50% off load once a minute will only ever reach half the calculated temperature rise, and that time is the most significant factor in diversity calculations. If my shower (12kW) has a 4mm2 cable, what will happen? I shower for 5 minutes and then it is off for 4 hours before it is used again. What is the cable maximum temperature? How hot does it get after my wife uses it for 15 minutes? Use the clipped direct ratings and resistance. If I have two of these showers in my house will the DNO fuse (100A) or even 60A, blow with showers as above at the same time? Just a little homework for the afflicted. This could be the most important lesson you will ever get. Just in case someone gets the wrong idea, I am not suggesting that the tabulated cable ratings should not be used for design, but the basis of them as continuous ratings should be understood, and the same with fuses and other CPDs.
I will answer another point Broadgauge, the appliance maximum power is less than it used to be because EU regulations are attempting "power saving" again. As for testers failing at 13A, what do they do with a 5W LED lamp? Do they still take 13A to issue a fail or do they know what was plugged in? Perhaps the 13A is because the tester cannot be CE marked if it has a 13A plug? Clearly another fools paradise.
AJJewsbury 77361768
1611 Posts
Chris Pearson:
broadgage:
It would appear that most appliance manufacturers take a similar view, as most new appliances are limited to about 10 amps, possibly due to warranty claims or adverse publicity about melted plugs.
3Kw heaters are now hard to find as are 3Kw tumble dryers. 

I think that it is more likely to be due to energy saving. IIRC, vacuum cleaners now have a statutory (EU) maximum power.
 

Or just marketing related - some countries traditionally rated their sockets at just 10A - so if you want to sell basically the same appliance across a wider market you need work to the lowest common denominator.
   - Andy.

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