Fridge & Freezer Radial - B or C curve on RCBO?

[axt123]

Hello,

Please help settle a mental ponderance.

What is the collective opinion on curve ratings on a 20A RCBO feeding two Radial feeds in 2.5 T&E, one going to a dedicated under counter socket (via DP isolator above counter) for a Freezer and the other doing the same but for a full height fridge. Could this sensibily be a C curve - in case both fire up their compressors simultaniously, or would this be silly and a vanilla B curve is ample / safer.

Obviously this is a circuit you would not want to nusance trip. It is also only domestic fridges / freezers so 20A B curve should do the job, but I can't shake the idea that when cooking you are likely opening and closing the doors fairly regularly and hence the opportunity for both to start together is heightened?

Should the radials be seperate, but then the old argument that if the freezer trips, would anyone notice? Plus this eats another way in the CU and seems a bit excessive really...

For that matter, is there in fact any call for a C curve MCB / RCBO in a domestic dwelling nowadays that incandescent & LV transformer lighting is mostly out the door? Garage Door Opener? A/C Unit? Boiler?

Many thanks for your thoughts...!

 

[Tom256]

Hello,

Please help settle a mental ponderance.

What is the collective opinion on curve ratings on a 20A RCBO feeding two Radial feeds in 2.5 T&E, one going to a dedicated under counter socket (via DP isolator above counter) for a Freezer and the other doing the same but for a full height fridge. Could this sensibily be a C curve - in case both fire up their compressors simultaniously, or would this be silly and a vanilla B curve is ample / safer.

Obviously this is a circuit you would not want to nusance trip. It is also only domestic fridges / freezers so 20A B curve should do the job, but I can't shake the idea that when cooking you are likely opening and closing the doors fairly regularly and hence the opportunity for both to start together is heightened?

Should the radials be seperate, but then the old argument that if the freezer trips, would anyone notice? Plus this eats another way in the CU and seems a bit excessive really...

For that matter, is there in fact any call for a C curve MCB / RCBO in a domestic dwelling nowadays that incandescent & LV transformer lighting is mostly out the door? Garage Door Opener? A/C Unit? Boiler?

Many thanks for your thoughts...!

No need to guess you can calculate - What is the inrush peak load current ?

You need to set by calculation the inrush peak load current to the minimum tripping current of the device. You can obtain exact values from device manufaturers or you can take the lowest multiple for the generic range.

You then take device rating {In} x instant trip lowest multiple x 1.414 (sq root of 2) and compare that against peak current so that peak load current < minimum instantaneous trip current.

That said 1 x domestic fridge and 1 x domestic freezer on a B curve 20A BS EN 61009 , you'll be fine

 

[axt123]

No need to guess you can calculate - What is the inrush peak load current ?

You need to set by calculation the inrush peak load current to the minimum tripping current of the device. You can obtain exact values from device manufaturers or you can take the lowest multiple for the generic range.

You then take device rating {In} x instant trip lowest multiple x 1.414 (sq root of 2) and compare that against peak current so that peak load current < minimum instantaneous trip current.

When you say minimum tripping current of device, do you mean the RCBO?

Hence for the 3-5x or 5-10x for the different curves instantanious trip this would be 60A and 100A for a 20A RCBO at the thin end of the wedge for instantanious tripping.

Some googling would suggest a fairly sanguine inrush current of ~2-3x running current for a fractional horsepower AC motor (typically ~500W), so seemingly under 10 - 12 A and a combined worst case should still be well below 60A (even multiplying by root 2 to get peak from RMS) for a B20. Hence my instinct would be to say a B curve is likely the norm for this.

However, as this is a thoretical fridge and freezer which may have a theoretical auto defrost heater as well I was not sure on the usual yardstick employed in the field when fitting a radial for an unspecified freezer socket, which was my general question really. Is a C curve ever seen in the wild being used for domestic refridgeration circuits?

Almost all guidance on C curve MCBs cite motors and compressors so it is logical to think about refrigeration as being an good candidate but I would think a C curve could comprimise safety if a compressor seized on startup for instance and didn't pull enough to trip the instantanious overload...

 

[westward10]

Any curve can be used in a domestic setting there is no such thing as a domestic protective device.

 

[Tom256]

When you say minimum tripping current of device, do you mean the RCBO?

yes the BS EN 61009 RCBO.
In a domestic environment standard circuit design [as set out] is a safe go to assuming all parameters are complied with. If however, you are installing any circuit for a specific [unique] load that may take you outside of such standard circuits you design accordingly. Such design includes overcurrent protection be that fault current - overload or both.

Protective devices be it BS EN 60898 or BS EN 61009 are suitable for household and similar applications, wide range, design accordingly.
Would such see a C curve installed? - if design requires such and all other parameters are correct then yes.
You cannot design around 'what if someday someone...' That would be for the Electrician handling that upgrade.

 

[Tom256]

Protective devices be it BS EN 60898 or BS EN 61009 are suitable for household and similar applications, wide range, design accordingly.
Would such see a C curve installed? - if design requires such and all other parameters are correct then yes.
You cannot design around 'what if someday someone...' That would be for the Electrician handling that upgrade.

Reminds me of a lad just completed his qualifications who believed the 'C' on the 60898 stood for 'Commercial'

You have to be patient at times I guess

 

[axt123]

Any curve can be used in a domestic setting there is no such thing as a domestic protective device.

Thanks, I guess I was just curiours to see what situations someone would use a C curve in a modern domestic setting as they seem well stocked with suppliers and equally available in all ranges...

In the past I used to see C curve (and even D) used in large domestic / light office use but mainly C6 and D6 on fluorescent lighting so when someone switches all the switches at once it doesnt trip the breaker...

Fridge / Freezer circuits seemed a possible candidate to my mind being motor based and something you dont want to erroniously trip...

Cheers.

 

[westward10]

Before the advent of LED lighting I always put type C on domestic lighting circuits as tungsten lamps often tripped a type B 6A when they failed.

 

[IAmSparkytus!]

Before the advent of LED lighting I always put type C on domestic lighting circuits as tungsten lamps often tripped a type B 6A when they failed.

Before the advent of LED lighting, I always put type B on commercial lighting circuits but now always put C

 

[Simon47]

My take would be :
Does the circuit support the use of a C curve device - i.e. would disconnect times be met ?
If the answer is yes, then you can use a C curve device. If the answer is no, then you can't.
If you can use a C curve, then the margin againt nuisance tripping will be higher. When my mother bought her bungalow, and it was rewired prior to her moving in, the garage was put on a B20 RCBO. It tripped more often that it would start a handheld cicular saw - but the numbers said a C curve was OK and that sorted the problem.

 

[IAmSparkytus!]

Would be very rare to find a domestic fridge which pulls enough when it kicks in to trip a 16/20amp breaker

 

[axt123]

Would be very rare to find a domestic fridge which pulls enough when it kicks in to trip a 16/20amp breaker

Indeed, that was my thought, a B20 having a 60A instantanious trip should easily cover a inrush for a domestic fridge, and if the fridge and freezer compressors both start in the same 10ms instant, so the same inrush curve, then you have won the trip lottery.