Should MCB's & RCBO's be Spaced Out in a Consumer Unit?
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- เผยแพร่เมื่อ 15 ก.ค. 2024
- You may have noticed a trend on social media for electricians to space out the circuit protective devices in a consumer unit due to something called Thermal Derating. Why are they doing this? Is it necessary? Should you be doing this whenever you install an MCB or an RCBO?
Join Original Joe as he unpicks this trend and considers guidance from the IET on the subject.
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🕐 Chapters 🕕
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00:00 Should Protective Devices be Spaced Out in a Consumer Unit?
00:14 Full free CPD Module on Circuit Protection: 👉 training.efixx.co.uk/course/c...
00:25 Spaced Out Trend.
00:46 What is Thermal Derating?
00:58 How does an MCB monitor overcurrent?
01:23 Other sources of heat and their effect.
01: 58 Is it that big a problem?
02:33 Do you need to space out the breakers for every installation?
02:53 A critical point and IET guidance.
03:12 Intermittent vs Continuous Loads
03:44 The circuits that may need spacing out
04:44 Loading Factors and Rated Diversity Factors
05:36 A worked example of a thermally derated circuit
07:03 How can we avoid thermal derating?
07:22 Do MCBs need installing from largest to smallest?
08:24 Next video in the series: 👉 • Warning: Potential Con...
08:25 Full free CPD Module on Circuit Protection: 👉 training.efixx.co.uk/course/c...
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📹 Presented by
Joe Robinson - Technical Editor
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#electricaldesign #electricians #electricalinstallation - วิทยาศาสตร์และเทคโนโลยี
I had a fault due to heat build up a few years ago. Small Office, quite a few people, IT equipment, Photocopier, fluorescent lighting etc. Lights MCB kept tripping, some days many times. Then maybe not for a couple of weeks. On investigating I realised the air conditioning was running and the MCB quite warm. It was next to the lighting circuit, and it was a period of very hot weather. The lighting circuit was also quite well loaded. Cooler days the air con didn't draw as much power, and there were no problems. Once I was satisfied this was the likely cause of the problem, changing the order of a few circuits & spacing solved it.
What a brilliant comment and experience, thanks Richard, great problem-solving skills 👍
After loading up CUs for the last 30 years now you tell me I have been doing it wrong. Or is it the MCBs made with far cheaper materials. You decide.
If we start doing it like this we’re going to need a full wall for a dis board
Wait until they make the generally pointless (in the UK) arc fault detector mandatory...
Forgive me if I seem to be overthinking this but Would it not seem like sense to fit solar panels and ground force heat pumps and all the other shit as standard practice on all new builds
Good video and I'm glad you took time to explain the different factors at work. For most older installations this would rarely be a problem, I've seen plenty of older electric heating boards where the circuits were in close proximity and would be on for long periods and n ever had an issue.
But with Solar, Electric vehicles and Electric heat pumps likely top be in most homes, not to mention many of these loads being most active at warmer times of the year. I would imagine this is going to become more problematic in future.
Thanks very much, glad you enjoyed it! 😊
Thanks Joe, useful to get this out there to more electricians. I use the spare blanks between high load CCTs where available such as on a CU change incorporating spares for future-proofing.Or, where there are none, I put B6s or little used socket circuits like a garage, next to the high load CCTs. Let's face it, lighting circuits in a modern home using LEDs will never warm up even if every lamp in the house is on 24/7 and apart from a kitchen socket circuit, do many homes use anywhere near the 32A ever, let alone for such a length of time that it will cause thermal issues?.... which leads me onto maximum load 😉
Great video Joe
Thank you sir! 😊
When I assembled my Hager board, I put them in in pairs with a gap between where reasonable - and I did not worry too much about 6A RCBOs.
Given that electricians have been installing MCBs in CUs for decades without worrying about this, then I do have to wonder whether this is a real issue of not. However, running larger cross-sectional cables is rarely a bad thing. It's something of an energy saver.
As far as RCBOs go, the electronics generate heat of the order of a 1 watt or so each. But 15 in a CU and that's 15 watts, and I do wonder if that will shorten the life of the RCBOs. Electronics generally last longer when they are cooler.
Some fair points, it's probably more about getting ahead of the problem now because loads are changing in people's properties so much. 🤔
Quality devices shouldn't have any long term heat issues. Even the cheaper end of the market, but still a quality device, that use low value electrolytic capacitors (1uF etc) should still last a long time with the expected temperatures in a crowded board. The electronic ripple controller (IC, film capacitors, MOVs etc) on my HWS is 30+ years old and shows no signs of incorrect operation. The metal meterbox is on an exposed wall and gets blasted by full tropical sunlight for hours each day. I should stick a thermometer in there and see how hot it actually gets.
It's more about the effect of the aggregated heat on the bi-metallic strip in a protective device. 🤔
As someone with a Car Charger on a 40A RCBO in the same board as a 63A MCB used to power 2 inverters for battery charging, I can tell you when the neutral wire heats up from the RCBO for the car charger and this heats the board and air inside, my 63A MCB goes from tripping at around 70A sustained load down to only around 61A.
The cabling on both the charger and inverter are rated for more than their protective devices so no danger running this high, but it did cause me to have to reconfigure the inverter power limits to below the 63A of the breaker, which when you look at the trip curves and it shouldn't be tripping at room temperature until around 70A+ on a continuous load, I was not expecting.
When the car charger isn't on the board stays much cooler and this issue doesn't occur even when intentionally testing the inverter circuit at over 68A for over an hour.
The RCBO and MCB are spaced out in here too, its just the RCBO neutral wire producing most of the heat.
@@piers19891 That's a fairly extreme example, especially if it it's on a 100A single phase supply.
7:34 Not in the UK, but i have been taught this as well.
The idea is that higher loaded single phase circuits are spread evenly across all three phases when using an alternating distribution/bus bar (not sure how it's called in English).
I do fit spacing blanks between rcbos but I tend to do it in sets of 2, fusebox price differences between the sizes are nominal
Is that on all circuits or just ones with sustained loads running close to the nominal rating of the breaker? 🤔
Great video and point taken. I do fit quite a few charge points and will consider the spacing on my next install. Be interested to see what a 7kw charge point's impact is on when next to another mcb.
With my industrial head on, at work all panels are wired with contactors sat side by side with continuous loads running for hours on end, wouldn't that be the same rules? I guess a contactor can take more continuous loads than an mcb?
On the contactor front, unless there's a thermal overload on the bottom of them then they shouldn't affect each other too much. It's not about the current the MCBs experience, it's the impact of the heat on the bi-metallic strip. Also contractors tend to be wider allowing for better heat dissipation. I'd need to look into contactors more to be sure to be fair. 🤔
Having just replaced some (domestic) contactors which were showing signs of heat damage, I would say spacing contactors is an especially good idea. The contactor coil generates heat regardless of the current being passed through it. Half module spacers exist specifically for cooling purposes.
Nice one thanks Jon. 💪
Great, I assumed that they were spaced out to make it easier to add a new circuit and maintain the decending order practise, I now know better thanks!
You're very welcome. 😊
i dont belive it for a second 😂
Which bit? 🤔
He morel of the story is don’t load the rcbo/ mcb to near it’s maximum rating. As my old boss used to say if there is any doubt put in the next size cable in and allow for a slightly higher MCB as you never know what’s going to be added to a circuit at a later date
30 amp fuse wire in Wylex fuse boxes never had this trouble!
I would space out anything i knew was going to be under full load often - eg . car charger.
Full size MCB/RCBO are being replaced by minuture versions. The outcome MCB/RCBO are getting hotter and trip times and currents are effected. This needs to be looked at, at the design stage by the manufacture.
eg CUs with 40 or 50 or 60Amp breakers are becoming the norm for UK Domestic installs with PV, EV. If the small form factor gets too hot maybe the manufacture could recommend the full height version and explain why it is needed.
A 12 way CU with spacers enbetween
Does look nicer
Can be dressed neater
Allows lots of room for inspection and testing
Does cost slightly more than a 8way unit
On the other hand a multi row or stack board gives you all the above but costs more and takes up a lot of space. If you were to consider it a replacement for a existing CU under the stairs or a new install in a new build. There would be different design considerations. Space and cost will probably the biggest issue.
Wonder if this will be the new 19th edition Regulation, to make us all buy a new board.
If a breaker can't be placed next to another then it's designed wrong.
They can be, but if they're feeding a sustained load then thermal derating may need to be applied. It's just physics to be fair. 👍
@@efixx Breakers' overheating from heavy loads easy to eliminate by increasing price for electricity even more.
Rather cynical point of view because we know there is never a requirement to bring installations up to the latest BS7671 standard, so you don't have to rip out existing CUs.
It will only apply to the installation of new CUs.
@deang5622 read who writes your regulations :) most councils/testers interpret that installs should comply with the latest regs
Interesting how its only done by OCD domestic sparks, Never seen a commercial spark do it on a 3 phase board? Perhaps these domestic sparks have far too much spare time?
Could be those three phase installs don't have sustained loads close to the nominal rating of the protective devices... or it could be the OCD thing! 😂
That seems completely obvious now you've mentioned it. But rather than leave a whole width space is there way to leave a half space, if they had half-sized blanking spacers. Is there evidence to know if this is a real problem or a potential problem which has rarely been seen?
I guess the half spacing would be an issue for connecting the next MCB to the busbar. 🤔 I think it's more based on the rise in the number of sustained heavy loads and an increased awareness of the possible risk. The thermal derating figures wouldn't be tested and stated otherwise. 👍
You've got a busbar problem there. And it's difficult to pre engineer a staggered spacing in the board at manufacturing time, as then 2 module devices wouldn't fit.
I thought this was the reason we're not allowed to mix different MCB brands - the heat dissipation hasn't been tested with differing brands touching.
Perhaps it's okay to mix brands if there's a blank between them?
I don't see why not. An RCBO or MCB knows nothing about any other protective devices in the consumer unit. The performance is as stated on the tin...it doesn't change because of other devices.
Wouldn't there be an option for changing our current practice of putting the highest loads nearest to alternating hi and low loads, oven, lights, heating, doorbell, etc
Yes, very much like we described at the end of the video. 😊👍
Do you mean going lowest rated circuit nearest the main SW and then going up in rating as it went away from there main SW? Ive thought this for a while, seems weird to try and cram the biggest cables in the area with least room, only for tiny 1mm/1.5mm cables basking in a huge amount of space
Probably better to alternate sustained loads between non sustained loads. 👍
@@efixx oh agreed! 👍👍👍
With my lot, you’re lucky if you get a spare way in a new DB 🤦🏻♂️☹️
My argument would be if the device needs to dissipate more heat then it should be of a larger 2 module design not one module. This would give it more elbow room out of the box, it could even have air channels built in to allow airflow. With a notch in the casing to fit over the not needed second line busbar terminal it could be marketed as suitable for constant current use and be ideally suited to EV type long duration high current loads. There's been a lot of discussion lately about ambient temperature affecting constant or long duration high current loads like an EV charger and over-current protection (eg consumer unit in an outdoors building having a lower/higher ambient temperature messing up the calibration of the thermal over-current part of a protective device). In some situations the operating temperature variations are going to be too large to accommodate depending on the seasonal ambient temperature. Especially on car chargers with the growing popularity of externally IP rated consumer units for EV installations. As a side note the only time I had a random RCBO operation was for my workshop and was due to a defective device. No fault was found until I dug out the thermal imaging camera and noticed it was running hotter than adjacent devices on virtually no load. Changed it out no further issue and normal operating temperature on the camera.
Yeah, but you can achieve the same thing without a specialised device by spacing or placing next to MCBs feeding low current intermittent loads. It's not necessary for every circuit just sustained loads running close to the nominal rating of the device. 👍
Slightly off topic but which Thermal Imaging camera were you using in this video please?
For this video we used a HikMicro thermal camera - don’t have the exact model number to hand. We also use an FLIR one which plugs in to iPhone etc from time to time.
we've historically mixed higher rating OCPD's with lower ones to dissipate the heat better, keeping all the higher rating devices collectively together is asking for trouble
Type B trip at 3 to 5 times the rated current. The heat would reduce this, would this not make it safer as it would trip at a lower current?
That is a good question.
The protective device doesn't trip at that current, it trips at a certain temperature which *_is_* caused by the current which causes a delta in temperature, above the baseline operating temperature of the protective device.
So if the cable is rated for 80°C, then that protective device is designed to trip at 80°C or less. Let's go with 80 for the purposes of this discussion.
So it doesn't matter what the ambient temperature is, that device is still going to trip at 80°C and irrespective of the magnitude of the current passing through it.
So it doesn't matter if the ambient temperature is 20 or 60°C.
If it always trips at 80° then it is clearly doing its job and protecting the cable, it is therefore no safer whether there is 10 amps passing down the cable or 20 amps.
The downside is, as the baseline temperature with zero current passing in the circuit, you are more likely to experience nuisance tripping because the full rated current for the circuit will push the protective device up over 80°.
customer question.
Can an old CU and supply (the master fuse from the supplier) be upgraded to a higher amperage when a new CU is installed? ie, from 60 or 80Amps to 100Amps.
You need to talk to the DNO.
Because it depends on the cross sectional area of the main service cable coming into the property through the main fuse.
That cable might not be thick enough.
@@deang5622 thanks.
If devices are manufactured to a BS or EN standard then it’s the manufacturer that should give what aspect of their standard of their standard does not meet the criteria and why separation is required.
It's compliance with the relevant standard that MIGHT lead to a need for spacing.
Like to view the circuit board with the thermo camera by default. Also step two is a check of connections on Torx with a moment-tool. Educate the customer, and maybe try to distribute the load more evenly.
Thanks for commenting. 😊
No. You should not need to do any redistribution of the load.
If the circuit carries X amps then the RCBO or MCB should be rated to carry that current.
You shouldn't need to divvy the load up and have multiple additional circuits with all the recabling costs and inconvenience that go with that.
You may need to deal with the over heating issue by increasing the ventilation around the protective device.
To adopt the solution of more circuits means more protective devices in the consumer unit and almost certainly reducing the airflow and worsening the over heating problem.
I suspect it will be more beneficial to leave a gap next to the protective devices which are at risk of overheating.
How long till we see CU's with a fan fitted ??
This is a good idea 😂😂😂
Just a few holes top and bottom would've been enough tbh, even small draft of air helps a lot.
im dying on the highest rated to main switch im old and i wont change so blanks it is XD
Forget spacing them, for continuous loads just fit a sub board when needed for solar and EV charge points. It’s best practice to do this and eliminates the cause of any problems !
How is that best practice?
Don't be daft.
And they are all connected to the same din rail (the main heat sink )
Don't think the din rail is a major issue really. 😊
😮😮
The thermal de-rating inside a breaker box is essentially only a function of the maximum current that can flow through the breakers in the box. That in turn is a function of only two things: (1) The main breaker amperage and (2) Whether the sub-breakers are 1-pole or 2-pole (or 3-pole, etc). And that is it. It doesn't really matter whether the high-load breakers are next to each other or not, and the number of breakers doesn't matter either (since the total is bounded by the size of the main breaker). The heat will effectively spread through the whole of the enclosure. More on that in a moment.
The third thing to remember is that the heat generated is a function of the square of the current. I*I. Because it goes by the square of the current, just using the normal rule of 20% for circuit ratings verses heavy or continuous loads essentially removes the issue entirely for nearly ALL situations. Current just 20% down vs the breaker size will massively reduce the heat generated inside the breaker... it will cut it almost in half, in fact.
The 20% rule can simply be applied across the board, whether the load is heavy, continuous, or not. Just follow the rule as a matter of course and you will be fine.
So, for example, if you have a 50A circuit and breaker for charging an EV, the maximum allowed EV charging current will be 40A. (50 * 0.8). If you stick to this rule, then you should not have to worry about breaker adjacency or spacing inside the breaker box. The adjacency is not going to effect the bi-metalic strips all that much, if at all. And even if it does a little, the actual current has already been de-rated to 0.8x the size of the breaker. So it's a non-issue.
Thermal de-rating in cable bundles and conduits is far more important than thermal de-rating in breaker boxes.
-Matt
No, your theory contains errors.
1. The heat is not uniformly distributed in a CU.
Those breakers carrying a higher load current are going to get hotter than those carrying a smaller current.
Whilst the heat will spread through conduction to devices in contact and through convection of the air inside the CU the fact is there is going to be a temperature gradient with the hottest point being on that higher current breaker(s).
2. You say that the thermal de-rating depends only on the maximum current that can flow through the breakers.
No, definitely not correct.
There is an ambient or baseline temperature. When current passes through the breaker that current heats up the breaker and that increase in temperature is a delta on top of the ambient baseline temperature.
The bimetallic strip in the protective devices detects an absolute value of temperature and trips when the threshold temperature is reached.
So if you have an ambient temperature of 20°C and the breaker trips at 80°C then you have 60°C of margin.
If the current is 20 amps and that creates a delta of 50°C then that causes the breaker to heat up from 20 to 70° - no problem.
But now if the ambient temperature is 40°C, that same 20 amps of current is going to push the temperature of the breaker from 40 to 90°C and now the breaker is going to trip.
So clearly, the idea that the ambient temperature does not have an effect is plainly and patently wrong.
So the maximum operating current has to be reduced (derated) according to the ambient temperature, otherwise you are going to end up with nuisance tripping and being unable to power the same number of devices, the same load in summer as you were running in winter.
3. Air flow around the RCBO and MCBs.
Having no space between the protective devices means that there is insufficient airflow to take the heat away and consequently the protective devices will heat up even more.
I have personally seen this with a stack of 3 external hard disks in plastic cases which were in contact with each other. The disk in the middle failed because it overheated. The others did not fail.
Clearly the close proximity of adjacent devices, if too close, are going to cause an increase in temperature rise of the target device and may cause nuisance tripping when high currents are being carried.
So again, this is another example where the derating of load current for protective devices should occur because of the layout inside the CU, providing another example disproving the assertion that it only depends on the current and nothing else.
@@deang5622 The de-rating has already been done. You are trying to do it twice to justify adding spacing that really isn't needed.
While all of these thermal issues are present to one degree or another, they are ALREADY baked-in to the standard de-rating that all electricians must follow for continuous load circuits. You don't have to do anything extra.
Also, I will point out that ambient temperature derating curves ARE NOT LINEAR. Circuit breakers are ALREADY rated for proper operation up to 40C ambient, and the derating curve above 40C is sub-linear and ALREADY fits within the standard 20% (125% rule) for continuous power.
Only extreme environments actually require extra de-rating. A breaker box inside the home isn't going to be in an extreme environment.
-Matt
@@junkerzn7312 So if you buy a 32 amp MCB and you wire that in, if the derating has been done then that breaker will be fine in UK up to 38°C and also will still handle 32 amps in Dubai at 50°C ?
Are you sure about that?
@@deang5622 You didn't read what I posted above about required de-ratings for continuous amps. Try reading it again. In fact, I don't think you are reading much of anything that I posted, or you wouldn't be asking such a stupid question.
-Matt
This is the same pretend problems we had with the 16th edition where people were earthing everything frrom meta lwindow frames and dorr letter boxes. You should be adviseing people about the buffoonery of ring mains!
Don’t forget the radiators and bed frames.
But the max rated temperature on MCBOs (at least for Eaton) is like 70C, with derating of 0.5% per C.So going from 25C to 40C only lowers it by 7.5%
7.5% is considerable for an Ev charger which is fully loaded for hours.
@@efixx I guess, here on continent we don't really get 40A single circuits, even 20A is pretty uncommon, as most higher power stuff runs on 3 phase.
What's the point in spacing the breakers if people are then going to tie the conductors together to take the perfect social media CU picture.
They're similar but separate issues to be fair. 🤔
If you put one of those full size blanks between them then you aren't allowing the breaker that is generating a bit of heat to dissipate anyway, be better to use a metal blank on the cover instead and allow the MCB/RCBO to sit in free air.
Qualified spark .you tube. Recently...says this spacing thing is nonsense...on higher load circuits...eg. 32 and higher ... And he was quite adamant on it ?????
It's not really either nonsense or mandatory on every install, it's something to think about on circuits with sustained loads close to the nominal rating. 🤔
Indeed, better safe than sorry. @efixx
Isn't this the reason we apply the 125% rule involving continuous circuits ie pv,battery storage or ev charge point
Correct, for any continuous load. Max-load = 80% x breaker-size, which is the same thing as saying max-load x 125%
I find having massive boards to accommodate this spacing is a bit excessive. what I do is alternate between "high' and low load circuits.
basically I do lights, cooker, lights, Ev, boiler, sockets etc etc. so, 6/32/6/32/6/32.
for apprentices, they'll find that is correct in the case of hager distribution boards which don't offer the same amperage at top of the busbar, as they do at the bottom. whereas schneider's busbar is rate 250A top to bottom for example.
Great comment, similar to what we suggested at the end there. 👍
Iv been mixing up breaker sizes for years. Is it just me or are we inventing new problems?
creating an issue that doesn't really exist
Let’s see how this plays out with higher adoption of solar, EV & heat pumps
Dutch rules are pretty far on power.
A bit to far 😄
Thanks for commenting! 😃
LOAD OF BOLLOX
This is all stuff you learn as an apprentice here in Denmark.
If youre installing cheaper branded gear that specify they need to be spaced out to prevent them over heating then sure.
Personally ill stick to brands than engineer their products better
It’s an issue with all brands.
electrical.theiet.org/media/3068/wiring-matters-issue-96-july-2023.pdf
@efixx yes, as it even says there, when constantly loaded. How many circuits can you say are constantly loaded to even 50% in a domestic setup?
but it's in a sealed unit, any heat created by MCBs would thermally transfer through the surrounding stagnant air, and much of it would transfer via the bus bar. I don't see a need to space them out.
Yes it would transfer through the air, but it comes down to things:
1. Is it a sealed unit and how well the air can escape from it
2. How good the thermal conductivity is from the breaker to the surrounding air. Two sides of that breaker are in contact with other breakers and the material is plastic, which is not a good conductor of heat.
The ability of that breaker to dissipate heat very much does depend on what is situated around it. If other plastic case devices are in contact with it, the total thermal resistance is going to be quite high and much higher than if there was a complete air gap around 5 out of the 6 sides.
@@deang5622 Well going off regs a consumer units should be fully sealed including intumescent strips around cable entries so if there was a fire inside the cu it could potentially stop it spreading, my point is modern metal consumer units are pretty much sealed.
And plastics while not very thermally conductive will still heat up to ambient temperature given plastics have quite a high thermal capacity so it takes a long time to also get cooler, thus keeping the internal air temperature above ambient for a longer period.
@@Daniel37parker Sealed units are worse thermally.
And as for everything heating up to the same temperature, that isn't relevant.
What is important is the steady state temperature the breaker reaches after it has been passing current for a length of time.
It is *_that_* which triggers the bimetallic strip inside the breaker.
And the ambient temperature matters because that is the initial starting temperature when the breaker is passing no current.
The question is, what is the delta, the increase in temperature of the breaker caused by the maximum circuit current passing through the breaker (or other protective device).
That temperature increase caused by the current flow adds to the initial temperature of the breaker.
You can't make assumptions about how hot it is going to get. You can try and estimate it mathematically, which will be hard, or you can perform an experiment and measure it.
You can measure the delta, the change between the initial temperature with no current flow and the steady state temperature reached with all circuits under full load.
Even better if they are made of sterling silver - for even better safety, of course.
General public already can't afford upgrading their electrical installations. You pushing for even more spending without real improvement of said safety.
I don't think we're pushing for more spending just highlighting something that may be an issue in some cases. We actually suggested a method at the end of the video that didn't require any larger boards at all. 🤷
no and no. also get 3phase instead of dumb single phase loads
Ooookay?
In a domestic environment 😁
its for sticking conduit into the boxes etc, not for sniffing..
Why not make consumer units with a more spacious spacing for breakers... rather than cross examining the ifs and butt's...just apply prevention is better than cure ...