HELP me figure out RCBOs in Split-Phase

[rollingask]

Hello,

I have a hypothetical question (I'm an electrical engineer) to better understand RCBOs and how they are used in the USA. Let's say I would like to install RCBOs (or maybe just RCCBs) in my home to protect against residual currents. I am in the USA so my home has split-phase (two phases, neutral and ground). Here is a stock photo of a distribution panel just to ask my question:

1. I understand people can replace the small breakers in the middle of the picture for RCBOs (1P+1N for a 120V socket and 2P for a 240V socket). Am I correct?
2. Assuming yes, then I'd have residual protection but only for that one socket or room. I'd need dozens of RCBOs to protect the whole residence. Is there a way to place an RCBO right where the phases and neutral enter the box (towards the North in the picture) before it gets distributed to all the little black breakers? This way, it would trip if ANY socket in the home has residual current to Earth?
   I guess it would have to be something like a 2P+1N RCBO but all I find are 1P+1N and 2P RCBOs. Could you tell me what would work, maybe with some pictures or a datasheet?
3. If what I am suggesting is not done, why is that the case? It should work in theory I think.

Thank you so much for any help.

 

[RetiredNY]

1. I understand people can replace the small breakers in the middle of the picture for RCBOs (1P+1N for a 120V socket and 2P for a 240V socket). Am I correct?
2. Assuming yes, then I'd have residual protection but only for that one socket or room. I'd need dozens of RCBOs to protect the whole residence. Is there a way to place an RCBO right where the phases and neutral enter the box (towards the North in the picture) before it gets distributed to all the little black breakers? This way, it would trip if ANY socket in the home has residual current to Earth?
   I guess it would have to be something like a 2P+1N RCBO but all I find are 1P+1N and 2P RCBOs. Could you tell me what would work, maybe with some pictures or a datasheet?
3. If what I am suggesting is not done, why is that the case? It should work in theory I think.

Thank you so much for any help.

1-Yes.

2-No. In the US our RCD (We call them GFCI - "Ground Fault Circuit Interrupter") circuit breakers trip at 5 mA. I believe the EU is more around 30 mA. We do not want a ground fault to plunge the entire residence into darkness. Further, we generally prefer to use GFCI devices throughout the residence where the electrical code requires them. (Kitchens, bathrooms, basements, garages, outdoors) and not to GFCI protect casual receptacles in places not likely to fault to ground like bedrooms, living rooms, etc.

3- We can install GFCI breakers to protect a single receptacle or an entire circuit, it depends on what is on that circuit.
4-Modern codes require arc-fault protection on most residential circuits, and CAFCI (Combined arc and ground fault) although the mA threshold is higher than 5 mA, I'm not sure exactly where the threshold is.

4-Regarding #2, the single smaller breakers could be for single dedicated circuits for appliances such as a heating boiler, furnace, dishwasher, built-in microwave, bath ceiling heater, sump pump, but generally each of those small breakers will feed upwards to 14 devices of receptacle outlets and lighting outlets. Because our general circuits are 120 volts we're limited to 1800 watts load on a 15 amp breaker and #14 awg. wire, or 2400 watts load on a 20 amp breaker and #12 awg. wire. That's the reason U.S. panel boards have a lot more breakers than a straight 240 volt country.

Higher wattage appliances typically run on 240 volts and are connected between both "phases" rather than 1 phase and neutral. Typically heat pumps, central AC condensers, kitchen cooking ranges, electric water heaters, electric clothes dryers.

 

[rollingask]

1-Yes.

2-No. In the US our RCD (We call them GFCI - "Ground Fault Circuit Interrupter") circuit breakers trip at 5 mA. I believe the EU is more around 30 mA. We do not want a ground fault to plunge the entire residence into darkness. Further, we generally prefer to use GFCI devices throughout the residence where the electrical code requires them. (Kitchens, bathrooms, basements, garages, outdoors) and not to GFCI protect casual receptacles in places not likely to fault to ground like bedrooms, living rooms, etc.

3- We can install GFCI breakers to protect a single receptacle or an entire circuit, it depends on what is on that circuit.
4-Modern codes require arc-fault protection on most residential circuits, and CAFCI (Combined arc and ground fault) although the mA threshold is higher than 5 mA, I'm not sure exactly where the threshold is.

4-Regarding #2, the single smaller breakers could be for single dedicated circuits for appliances such as a heating boiler, furnace, dishwasher, built-in microwave, bath ceiling heater, sump pump, but generally each of those small breakers will feed upwards to 14 devices of receptacle outlets and lighting outlets. Because our general circuits are 120 volts we're limited to 1800 watts load on a 15 amp breaker and #14 awg. wire, or 2400 watts load on a 20 amp breaker and #12 awg. wire. That's the reason U.S. panel boards have a lot more breakers than a straight 240 volt country.

Higher wattage appliances typically run on 240 volts and are connected between both "phases" rather than 1 phase and neutral. Typically heat pumps, central AC condensers, kitchen cooking ranges, electric water heaters, electric clothes dryers.

Hello,
Thank you so much for your answer, it is very clear.
2) Ok so the reason is not that there's a fundamental problem, or a physics issue in terms of the toroid, or what the currents are like, that prevents this. It's just the inconvenience would be so large that in practice, people do not do it and hence, manufacturers do not seem to make it.

Just for curiosity's sake: Let's say I did want to protect the whole residence with just one device and I am OK with the inconvenience you described. What would that RCD look like? Would it be a 2P version where you feed both phases through, and leave Neutral outside of that RCD? Or would it be a 4P version with both phases and also neutral connected through it, and I leave the fourth pole just open and unused?

Thank you for your help.

 

[RetiredNY]

Hello,
Thank you so much for your answer, it is very clear.
2) Ok so the reason is not that there's a fundamental problem, or a physics issue in terms of the toroid, or what the currents are like, that prevents this. It's just the inconvenience would be so large that in practice, people do not do it and hence, manufacturers do not seem to make it.

Just for curiosity's sake: Let's say I did want to protect the whole residence with just one device and I am OK with the inconvenience you described. What would that RCD look like? Would it be a 2P version where you feed both phases through, and leave Neutral outside of that RCD? Or would it be a 4P version with both phases and also neutral connected through it, and I leave the fourth pole just open and unused?

Thank you for your help.

Manufacturers don't make it. Since our GFCI requirements are set a 5 mA, the culmination of combined natural mA leakage across so many circuits would lead to nuisance tripping of epic proportion. Main breakers are built to a physical frame different from the individual branch circuit frames.

We don't have main RCDs so I'm not about to hypothetically imagine what that might look like.

 

[westward10]

Are you American or a Brit based in the States as your terminology is more here than there.

 

[pc1966]

Just for curiosity's sake: Let's say I did want to protect the whole residence with just one device and I am OK with the inconvenience you described. What would that RCD look like? Would it be a 2P version where you feed both phases through, and leave Neutral outside of that RCD? Or would it be a 4P version with both phases and also neutral connected through it, and I leave the fourth pole just open and unused?

Any RCD has to sense all of the "live" conductors, those that normally carry current, so it can see if anything has gone astray. Hence neutral has to pass through along with all line conductors, and neutral must not be linked to earth/ground after the RCD.

Typically for a two phase system you would use a 4P RCD and leave one of the poles unused. However, you have to be careful about that as the self-test button on 4P RCD is usually between phases, and not L-N as you see for SP (2P) RCD since a more common situation (at least in UK/EU) is a 3P load without N, for example a motor. So check the manufacturer's data sheet about that.

It is consider poor practice to protect a whole installation against shock risk by means of an incoming RCD, as the risk of it tripping on accumulated leakage currents and you having no lights, etc, is quite high. However, in locations where TT earthing is employed (typically rural sites) it is common to have a higher current, but delayed-action incomer RCD for protection against faults as you are unlikely to disconnect on over current (fuse, MCB, etc) on a typical earth electrode's impedance.

The higher trip current (often 100mA or 300mA) and the short-term delay (typically 0.2s) allows selectivity with down stream RCD/RCBO used for personnel protection. So if you get a slow rising low fault on a circuit the RCBO trips before the higher threshold is reach, but if you get a sudden high fault current then the delay gives the down-stream RCD/RCBO time to disconnect before the upstream device decides action is needed.

In UK/EU that final circuit RCD/RCBO is usually 30mA trip (actual threshold is 15-30mA) and used to protect a whole circuit. In the USA the practice is for lower threshold and per-load protection, and in both cases they are "instant" in that they act in barely tens of milliseconds.

Now the arguments for 5-6mA (USA style) or 30mA (UK/EU) threshold are complex, but what you must always remember is an RCD does not limit the shock current. What it limits is exposure time to currents above the trip threshold. This is why you need alternative protection (such as very low voltage) in situations where high conductivity is present, such as bath/shower/swimming pool, as a short but very high current shock can still cause damage or death.

TL;DR is 30mA is a threshold that is low enough to prevent heart fibrillation in most cases, and high enough to have few trips on a typical final circuit with multiple loads. 10mA or less is to prevent the inability to let go for most people.

You have to be quite unlucky to get a sustained value in those regions: usually you withdraw your hand, etc, involuntary which can cause injuries from cuts to falling off ladders, or you grasp tighter as you no longer have real muscle control, and that increases the current leading to an RCD tripping quickly.

Neither is anything but painful!

 

[pc1966]

We don't have main RCDs so I'm not about to hypothetically imagine what that might look like.

This is an example for a UK/EU board 3-phase board:

https://www.cef.co.uk/catalogue/products/1540839-100a-4p-300ma-time-delayed-rcd-incomer-kit

More commonly it is just a 3P or 4P isolator switch for TN earthing:

https://www.cef.co.uk/catalogue/products/1540734-125a-tp-switch-disconnector-incomer-kit

 

[rollingask]

Any RCD has to sense all of the "live" conductors, those that normally carry current, so it can see if anything has gone astray. Hence neutral has to pass through along with all line conductors, and neutral must not be linked to earth/ground after the RCD.

Typically for a two phase system you would use a 4P RCD and leave one of the poles unused. However, you have to be careful about that as the self-test button on 4P RCD is usually between phases, and not L-N as you see for SP (2P) RCD since a more common situation (at least in UK/EU) is a 3P load without N, for example a motor. So check the manufacturer's data sheet about that.

It is consider poor practice to protect a whole installation against shock risk by means of an incoming RCD, as the risk of it tripping on accumulated leakage currents and you having no lights, etc, is quite high. However, in locations where TT earthing is employed (typically rural sites) it is common to have a higher current, but delayed-action incomer RCD for protection against faults as you are unlikely to disconnect on over current (fuse, MCB, etc) on a typical earth electrode's impedance.

The higher trip current (often 100mA or 300mA) and the short-term delay (typically 0.2s) allows selectivity with down stream RCD/RCBO used for personnel protection. So if you get a slow rising low fault on a circuit the RCBO trips before the higher threshold is reach, but if you get a sudden high fault current then the delay gives the down-stream RCD/RCBO time to disconnect before the upstream device decides action is needed.

In UK/EU that final circuit RCD/RCBO is usually 30mA trip (actual threshold is 15-30mA) and used to protect a whole circuit. In the USA the practice is for lower threshold and per-load protection, and in both cases they are "instant" in that they act in barely tens of milliseconds.

Now the arguments for 5-6mA (USA style) or 30mA (UK/EU) threshold are complex, but what you must always remember is an RCD does not limit the shock current. What it limits is exposure time to currents above the trip threshold. This is why you need alternative protection (such as very low voltage) in situations where high conductivity is present, such as bath/shower/swimming pool, as a short but very high current shock can still cause damage or death.

TL;DR is 30mA is a threshold that is low enough to prevent heart fibrillation in most cases, and high enough to have few trips on a typical final circuit with multiple loads. 10mA or less is to prevent the inability to let go for most people.

You have to be quite unlucky to get a sustained value in those regions: usually you withdraw your hand, etc, involuntary which can cause injuries from cuts to falling off ladders, or you grasp tighter as you no longer have real muscle control, and that increases the current leading to an RCD tripping quickly.

Neither is anything but painful!

Unbelievably helpful, thank you very much. Literally getting to the exact heart of my questions.

Also, it sounds like one could use a 3P RCD like the isolator switch for TN grounding you linked, with two phases and neutral right?

 

[pc1966]

Also, it sounds like one could use a 3P RCD like the isolator switch for TN grounding you linked, with two phases and neutral right?

You could, but it is unusual for TN earthing as usually you can disconnect on over-current. For smaller panels it is typically just an isolator switch as incomer, but bigger panels and/or if the upstream supply is not already limited to a sane level then an MCCB incomer (like a big MCB) is the typical alternative.

MCCB usually have a bit of short-term delay so they are reasonably selective with down-stream MCB, they can also have fancy electronic trips (some with RCD functionality) allowing specific trip characteristics to be programmed.

Here in the UK it is quite common for the supply to have HRC fuses, and so that is often adequately limited before you get to the panel.

In the UK a typical case where an RCD incomer is seen even with TN earthing is situations such as agricultural installations as a bit of fire protection against vermin chewing cables, etc.

The EU often features TT earthing, hence RCD incomers are more common there.

 

[RetiredNY]

Unbelievably helpful, thank you very much. Literally getting to the exact heart of my questions.

Also, it sounds like one could use a 3P RCD like the isolator switch for TN grounding you linked, with two phases and neutral right?

Here's how we generally protect circuits in the States, although GFCI breakers are also available.