13A Switch fuse spur

Question

Hello, Does anyone know if there are regulations stating that the 13A switch fuse spur can run Continuously at is rated current for long hours? I find this intriguing because, when a spur operates at its maximum rating, it tends to overheat, but i can't find any regulation that explicitly states this isn't allowed. The only regulation i can find in the 7671 concern installation cable size, etc. I know that electricians need to take cable size into account during installation but most of them just follow the standard, without considering the 80% rule from current capacity. the issue i have is that, although it's good practice, i can't find a regulation explicity stating this. So, i'm not sure what to rely on when it comes to the regulations. Does anyone have any insights??

Newfutile · Accepted Answer

im not sure i would want to run anything at its maximum for a long time. 
 good workmanship may well cover this, if you want to run a 13 amp load use a 16 or 20 amp mcb / rcbo with a 20 amp switch if needed locally.

broadgage · Answer

My personal view is that fused connection units should not be used for a long hour load of more than about 10 amps, say 2.3 or 2.4 Kw. That is however my personal view of good practice and not a regulatory requirement. 
 Equipment with a stated nominal loading of 10 amps could use up to 11 amps actual due to a combination of manufacturing tolerances and supply voltage fluctuations. 
 I would have little concern with short term or intermittent loading to a full 13 amps nominal, and perhaps 14.2 amps actual. 
 For long hour loads exceeding 10 amps, or short term loads exceeding 13 amps nominal, I would consider it best to avoid 13 amp fused connection units entirely, and instead use a dedicated radial circuit from a 16 amp or 20 amp MCB, and a local 20 amp switch if required.

aligarjon · Answer

The terminals in 13amp fused spurs and screw sizes generally are the same size as in 20 amp switches and double sockets. I suspect the issue is more the 13 amp fuse getting hot if run at full load rather than the switch terminals. 
 I came across a 3.4 kw storage heater plugged in a couple of years ago. I don't know how long it had been like it but the plug was welded in. They only called me out because it was pretty much on fire. 
 Gary

AJJewsbury · Answer

In general the ratings of things should normally be their continuos rating.. but some standards do have other ideas. I guess you'd need to refer to BS 1363 to see precisely what it demands. 
 The traditional way to overheat a 13A fused connection unit was to use it to supply a3kW (at 240V) immersion heater - although the real cause was the ambient temperature in the airing cupboard rather than the current - I think BS 1363 expects something like an average of no more than 25 degrees. 
 That said, 13A plugs/sockets have been known to cause problems when feeding EVs - there are now special "EV" marked sockets that have a true 13A continuous rating. 
 - Andy.

mapj1 · Answer

The british standard sets this limit, and running warm is indeed expected by design, partly due to the fuse warming up and partly the rest of it. How hot however is an interesting point. There are heating tests and accelerated ageing heat tests as part of the standard. https://knowledge.bsigroup.com/products/13-a-plugs-socket-outlets-adaptors-and-connection-units-13-a-fused-connection-units-switched-and-unswitched-specification?version=tracked 
 however at over &pound;400 you can understand why most folk probably don't have a copy to hand to check the exact figures. Mike.

gkenyon · Answer

Bruno Miguel Pereira said: Hello, Does anyone know if there are regulations stating that the 13A switch fuse spur can run Continuously at is rated current for long hours? I find this intriguing because, when a spur operates at its maximum rating, it tends to overheat, but i can't find any regulation that explicitly states this isn't allowed. 
 Not directly ... BUT ...Regulation 433.1 is, I think, where you need to go. Care is needed regarding how the rating of the wiring system and accessories relates to the rating of products when calculating load currents. 
 For example, BS EN 60335-1 permits current-using equipment to the standard to use 5 % more power (or current) than the stated rating - meaning a device which is rated 13 A may well use 5 % more load current (13.65 A) at nominal rating plate voltage. The utilization voltage also varies from between + 10 % (253 V) and - 11 % (204.7 V), using BS 7671 voltage drop maximum of nominal. With linear (resistive) loads, such as heaters, the load current increases with increasing voltage, and with switched-mode power loads the current increases with decreasing voltage to achieve the same power. What this all means, is that a heating load rated 3 kW at 230 V might actually have a load current up to 15.1 A (or as little as 10.4 A). Similarly, a heating load rated 3 kW at 240 V might have a load current as much as 13.8 A (or as little as 9.6 A). 
 Regulation 433.1 of BS 7671 states: 
 Every circuit shall be designed so that a small overload of long duration is unlikely to occur. 
 So, as we can see, even under normal conditions of temperature, a BS EN 60335-series product rated 13 A (2990 W) at either 230 V or 240 V perhaps should not be used for long periods from a device protected by a 13 A fuse. Immersion heaters are often rated 3 kW at 240 V, but have the added problem that, if the connected device is in the same room as the heated cylinder, the ambient temperature will be elevated, and wiring and accessories should be derated accordingly. This is noted in Section 5.3.1 of the IET Commentary on the Wiring Regulations. This is supported by BS 1363 series ... BS 1363-4, for example, has in its stated conditions of use for relevant products (connection units, or "spurs") that the maximum ambient temperature is permitted to get to 40 deg C, BUT that the average ambient temperature over 24 hours is not to exceed 25 deg C. Overall, where a current-using product has a rating exceeding 2.7 kW at 230 V, the rating of accessories for its connection, and any associated protective devices, should be higher than 13 A, and where elevated temperatures are expected, a much lower rating limit for products is necessary for the 13 A limit. I would, however, at this point, state that there is nothing to prevent a manufacturer exceeding the requirements of BS 1363, and I am in no way making a statement about the suitability of any and all accessories to BS 1363 series for continuous "rated at 13 A" or "rated at 3 kW" loads ... just considering the "minimum performance" that you might need to achieve a conformant product.

Chris Pearson · Answer

The answer is not simple. I have had a look at the 2018 version of BS 1363-4. 
 Ambient temperature -5&ordm;C to +40&ordm;C with an average not to exceed 25&ordm;C. 
 The type testing to ensure compliance uses a 20 A supply with a 14 A load on the output and a further 6 A on the input (presumably simulating a ring, or a load further along a radial). The ambient temperature is to be 20&ordm;C &plusmn; 5&ordm;C. A maximum temperature rise of 52&ordm;C is permitted after a minimum of 4 hours, but there is a caveat that an FCU is designed for a maximum cable temperature of 70&ordm;C. 
 Type testing is, however, not normal use. BS 1363 states that FCUs "... shall not attain excessive temperatures in normal use." Some manufacturers exceed the minimum standards, so if you want to run a long 13 A load, I think that unless the ambient temperature is no higher than 18&ordm;C, it would be wise to monitor the temperature, at least initially.

Chris Pearson · Answer

The answer is not simple. I have had a look at the 2018 version of BS 1363-4. 
 Ambient temperature -5&ordm;C to +40&ordm;C with an average not to exceed 25&ordm;C. 
 The type testing to ensure compliance uses a 20 A supply with a 14 A load on the output and a further 6 A on the input (presumably simulating a ring, or a load further along a radial). The ambient temperature is to be 20&ordm;C &plusmn; 5&ordm;C. A maximum temperature rise of 52&ordm;C is permitted after a minimum of 4 hours, but there is a caveat that an FCU is designed for a maximum cable temperature of 70&ordm;C. 
 Type testing is, however, not normal use. BS 1363 states that FCUs "... shall not attain excessive temperatures in normal use." Some manufacturers exceed the minimum standards, so if you want to run a long 13 A load, I think that unless the ambient temperature is no higher than 18&ordm;C, it would be wise to monitor the temperature, at least initially.

mapj1 · Answer

and realistically at 18C ambient and a unit under test 70C temperature -presumably on the outside where you can measure it, parts of the spur are quite a bit hotter inside. The plastics will age more rapidly and the whole thing may well have a short and uncomfortably hot life compared to running at half that load. Expect failure times measured in years rather than decades. 
 Or for sustained loads use a DIN box and an MCB or a 20A switch and protect elsewhere. Mike.

gkenyon · Answer

Chris Pearson said: The type testing to ensure compliance uses a 20 A supply with a 14 A load on the output and a further 6 A on the input Bruno Miguel Pereira said: Clause 17 of 1363-4:2023 specifies the operating temperature, and typically, the test is conducted for 8 hours with a 14A load and a 6A load , totaling 20A . :). i know the standards but thank you for your feed back. 
 Again, not that simple ... BS 1363-4 actually states 20 A nominal test current used for the type test are actually14 A &plusmn; 0.4 A on the load terminals, and a balance of load on supply terminals of 6 A &plusmn; 0.4 A. So, the test currents used for type testing may well be 13.6 A on the load terminals, and 6.6 A parallel load on the supply terminals. 
 gkenyon said: What this all means, is that a heating load rated 3 kW at 230 V might actually have a load current up to 15.1 A (or as little as 10.4 A). Similarly, a heating load rated 3 kW at 240 V might have a load current as much as 13.8 A (or as little as 9.6 A). 
 So, you see the type test of BS 1363-4 may not even provide sufficient current flowing through the load terminals (and fuse) for a given immersion heater rated 3 kW at 240 V, never mind one rated 3 kW at 230 V ... or indeed an appliance rated at 13 A at 230 V (which may, according to BS EN 60335-1, actually have a nominal load current at 230 V of 13.65 A).

13A Switch fuse spur

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