MCB Tripping - Overheating

jasgrif11

Hi,

I’m looking to get your thoughts on my setup. I do have an electrician calling to me within the next 2 weeks, but would be good to know some questions or ideas I can ask him.

My Setup:

I have a 16kVA supply, a large solar array, a large battery (40kWh), 2 X EVs and a heat pump. So yes, we are a large electrical usage home.

I pull down 17000-18000w constantly each night during my 4-hour cheap night rate. During this time I charge the battery (inverter pulls 8000w), I charge the car (Zappi pulls 7000w) and the house load makes up the rest. The heat pump is very spikey in that when it kicks in, it draws 5000w, and it tappers down to 3000w. I then run the house off the battery all day.

I have one main consumer unit and two sub-units (upstairs and garage). My inverter and battery are in the garage. The garage board has a 63A fuse, and the inverter is protected by a 40A RCBO.

My main consumer unit has an 80A MCB, a 63A breaker for the feed to the garage, and a 63A breaker for all downstairs sockets and lights.

My Issue:

I’ve had a few instances where my main 80A MCB tripped during the 4-hour charging window. I think this is due to overheating, and I think I tracked it down to the fact that there are two wires coming out of my main MCB to each of the 63A breakers, but one is 16mmsq, and the other is 10mmsq.

Should both be 16mmsq cables?

My house was built in 2008. Are there tests my electrician can run on my setup from a safety perspective?

Should my main MCB be a type C given the large load and spikey nature of the heat pump?

Should I upgrade any of the equipment or cables?

tomhammer..

10 and 6 wouldn't matter in DB

B63 Should be ok too

What make are those breakers

Never heard of them

jasgrif11

Noark is the brand. I’ve definitely seen them around

10-10-20

The heat off a wire won't cause a MCB to trip BTW, the MCB is supposed to be sized to protect the wire but it only does that by detecting an overload condition due to high amps, rather than the temp of the wire.

What did the CT on the incoming wire (ac grid supply) read when the power dropped? Do you have that logged?

10-10-20

Just to explain that breaker off the tech sheet:

https://cdn.noark-electric.eu/documents/e93c87a9-b282-45b3-8b35-bc0605c09de5/ex9b125-eu-en-20230111.pdf

For that breaker, I/in on the x-axis is x times the current rating.

So if your breaker was rated at 80A and you were drawing 80A across it, that would be equivalent to "1 I/in" and reading up from 1 on the x axis gives you a value of 10,000t/s. So for a load of 80A you would expect this breaker to drop after ~10,000 seconds.

For a load of 160A it would would be 2 I/in, so the tripping time would be expected to be between 15sec and 200sec, and so on.

So hypothetically you're seeing a drop after ~2000 seconds, so that could equate to a continuous load of ~1.15 times the rated current. That's assuming that the MCB is operating normally.

blackbox

MCB itself shouldn't get hot.

Maybe it should be replaced.

jasgrif11

The MCB itself wasn't hot. The 10mmsq wire feeding into it (the bottom) was much hotter than the 16mmsq wire feeding in. This could be the characteristics of the difference in the wire.

The MCB tripped at 3.57am with a total load of 17472W. It tripped again at 5.22am with a total load of 17636W.

I've overlayed the total grid + car + heat pump and added a zoomed-in view of it. I don't see anything significant here.

Anything else I should try here?

jasgrif11

@10-10-20 Is there a particulare brand of MCB that you would recommend or have they all similar characteristics? Am I just overreaching, thinking the heat pump spike is a potential issue? The Heatpump itself is on a 40A MCB.

meercat

What’s your supply voltage

have you 25mm main cables

jasgrif11

237.2 V and yes 25mm main cables

10-10-20

https://www.boards.ie/discussion/comment/121872480#Comment_121872480

No, you don't change the characteristic on the basis of a potential fault.

10-10-20

https://www.boards.ie/discussion/comment/121872461#Comment_121872461

Let me get this right...

Sorry if this seems remedial but I like to be sure: Is "Deye grid CT power" a combination of "heatpump" and "myenergi car" as well as the other house loads, or do we need to add "heatpump" and "myenergi car" to "Deye grid CT power" to calculate the total house load? So was the TOTAL house load 17636W at the time of the second trip or was it 17,636 + 5,000 (heatpump) + 5,000 (EV) = 27,636w?

Also confirm this for me... Does this brown wire loop like I drew?

jasgrif11

The total amount was 17636W for everything. I only overlayed the car and heatpump as they are probably my biggest loads after the battery during that period.

Yes the brown wire loops like you drew.

2 x 16mmsq wires now go from the main MCB to each of the 63A breakers. Then there is a 10mmsq cable that goes from one of the 63A to the Heatpump MCB just like you drew

10-10-20

https://www.boards.ie/discussion/comment/121873050#Comment_121873050

So different now from the photos? Previously the 40A and one of the 63A's were fed from the 10mm2 coming off the 80a MCB?

I presume you don't have the HA graphs for the periods where the MCB tripped? I'm thinking that you probably had an additional unexpected load on the MCB and it tripped on that basis. As to how to troubleshoot that... I guess that given that it's still only March that isolation of potential loads isn't going to be a popular suggestion...? 😉

jasgrif11

Yes different in that yesterday morning I removed the 10mmsq and fed each of the 63A breakers with 16mmsq cables. Lastnight there wasn't any MCB trips.

When you ask about HA graphs I only have the grid charts from the Deye CT and also the Zappi CT both of which are connected directly to the tails in the Meter box.

Here is a graph for the last 2 days. There was a trip (twice) on Friday night and none on Saturday night. That 6am negative on MArch 9th is genuine, I have a script to export based on battery percentage and weather. forecast.

tomhammer..

You're wasting effort upping cables in main board from 10 to 16 .Makes no difference.

Also quite dangerous for a non -rec to undertake this work.

There's a fair chance you've left a Timebomb too with connections not fully tightened . Very common for non sparks to do.

rob w

As others have said, the MCB is tripping because the current is too high for it. Cable size has nothing to do with that, it's the loads that are drawing the current....but if a 10mm sq cable was feeding a 63A MCB, that shouldnt be the case...to small a CSA, so thats why it's getting hot.

Do you have any graphs showing the actual current measured at the time of tripping?

You could be very close to the 80A limit when considering power in Watts only. That's assuming a power factor of unity, which it probably won't be with the loads you have running. This could be tipping the current over the limit and tripping the MCB eventually

Changing MCBs is not the thing to do here to resolve this. You need to make sure the load is managed appropriately based on the supply you have. So maybe that means dialling down the Zappi or battery charging a touch to keep you under the limit.

If you are concerned get a REC into to take a look. After all your loads are pushing the system to the limits here, cables heating up and MCBs tripping due to overload while you're asleep in the middle of the night. Fiddling around with connections in the board here might be a recipe for disaster if connections are left loose.

tomhammer..

..

jasgrif11

I have a REC calling to me during the week.

I thought the Shelly would measure Amps but it doesn't. Are there any sensors that do outside of a manual clamp meter? I suppose I'm trying to get to a place where I can maximise draw from the grid at night, but doing so safely is the absolute priority. I thought the Zappi is supposed to auto-dial down based on the rest of the load.

For now I have dialled back the battery charging a bit until the REC calls.

10-10-20

https://www.boards.ie/discussion/comment/121874033#Comment_121874033

Which CT is the shelly and where is it physically placed and what is it's data being used to load-control? Is that white one on the bottom of the 100A AC supply not the "Deye grid CT power"? Also you said this: "...Zappi CT both of which are connected directly to the tails in the Meter box", that's what's confusing me. Do you have any connections to any devices within the meter box that aren't being seen by the last CT in the consumer unit (fuse box)?

If you feel like uploading a line drawing of the meter box, consumer unit (showing major circuits/loads only), Deye, Myenergi and EV charger and then draw in where the individual CT's are, that might help.

In my mind you have too many import limiting devices which are independent loads and not enough overhead built into the draw off the grid. None of these devices are interlinked, each one works to manage the import independently of the rest and part of the problem is the resolution at which many of these CT based devices operate as some only see the grid-load only every 5 to 10 seconds. I know that the shelly is normally faster than that, maybe every second, but the asynchronous nature of independent import load management is what's probably causing the issue here as one device is possibly seeing load gaps and increasing it's consumption on a slow resolution while other devices are also making the same change within that same resolution period. 💥

10-10-20

I thought the Shelly would measure Amps but it doesn't. Are there any sensors that do outside of a manual clamp meter?

So what is the Shelly not doing? Surely if it's connected to a live wire, is the nearest CT to the meter and with the right CT - then it should measure amps?

A CT is the only way really. There are other meters out there such as hall-effect CT's (measure the magnetic flux) and these are more precise than wire-wound ferrite CT's, and there are also v-drop (resistance) meters but these are uncommon on AC systems (that I know of).

rob w

Yeah the Zappi should dial down based on grid current (I've never exceeded on my own supply to test the theory though), and that would obviously require the Zappi grid limit is set properly and that the Grid CT is 'seeing' all current as per poster above. Have you checked settings/CT location?

The resolution/real time of the CT readings another important point made above.

It's an interesting dilemma alright, as this is the way domestic electrical supplies are heading in the future.

jasgrif11

So first off I apologise for the crude drawing but hopefully you will get the idea.

1. There are 2 CTs on the Live tails coming out of the Smart meter in the meter cabinet. The first one is the Deye CT and this is wired directly into the Deye Inverter.
2. The second CT is the Zappi CT and again this is hardwired into the Zappi (not via a Harvi). I do have a Harvi but this is for monitoring the inverter output for Solar via the myenergy CT.
3. The Zappi grid limit is set at 80A
4. The Shelly CT was something that I added to the Live input to my main CU. I added this ever before I got solar and it's not controlling any load. It's purely for reporting. In fact I could remove it.

jasgrif11

Just as a quick test, I set the battery charging at 8000w, the car on fast mode (6700w) and then measured the current with a clamp meter at the outside meter location. It was measuring 63A.

The 63A included the standard house load at that time. There wasn’t anything else heavy running like a heatpump or oven.

Nothing tripped, im just highlighting what the current was.

I then put a thermal camera on the consumer unit. The MCB itself didn’t measure as hot. The line in to the MCB was 45 degrees and the line out of the 63A breaker to the garage measured 56 degrees (right at the top at the connection point).

10-10-20

https://www.boards.ie/discussion/comment/121874797#Comment_121874797

OK, that all looks sane.

So when the breaker dropped on Friday I presume you lost HA as it went offline? A potential hypothesis is that when then that could be why you don't see any spikes beyond 17,636kW as the data wasn't written down to disk. Could that be possible?

Otherwise if you're fully sure that the load didn't spike above 17,636w, then it indicates that the MCB could be faulty. Only measuring the current at the meter over an extended period and saving it down (using a logger etc) is going to show otherwise.

10-10-20

https://www.boards.ie/discussion/comment/121875064#Comment_121875064

If all those cables are now 16mm2 and you're seeing a 56 degree C hotspot only around the screw-terminal of that breaker, then I'd suggest you might have a bad breaker and/or cable. But it's still not going to be the trigger for the main 80A MCB to trip.

jasgrif11

Yes host HA was lost when the MCB tripped so not sure what spiked, but i’ve a feeling that’s what happened. It has only ever happened during the high load period.

I’ll see what the REC says when he calls.

jasgrif11

Thanks for the help