Bi directional RCBOs

[littlespark]

Fitted a new design Proteus board a few weeks ago, and one of there improvements is that all RCBOs are now biderectional, as well as SPD with built in fuse, no need for upfront MCB….

But is the bidirectional RCBO the way everyone is going now?

I understand for specific circuits, ie PV, battery storage and vehicle based battery storage… but what about regular circuits that are only downstream from the consumer unit?


Say you have a lighting circuit on a 6A RCBO and something develops a fault… Could this fault affect any other bidirectional RCBO as it would be detected coming from the busbar side?

Depending how sensitive the 6A is, that it doesn’t trip first.

 

[mainline]

Say you have a lighting circuit on a 6A RCBO and something develops a fault… Could this fault affect any other bidirectional RCBO as it would be detected coming from the busbar side?

What sort of fault are you thinking of, that would be on the bus bar.

 

[littlespark]

Any fault.... on another circuit might (if its own rcbo doesnt trip in time) will come through to the busbar, and may trip another, being sensed on the supply side rather than the load

 

[Pretty Mouth]

Say you have a lighting circuit on a 6A RCBO and something develops a fault… Could this fault affect any other bidirectional RCBO as it would be detected coming from the busbar side?

Won't pretend I'm an expert on this, as I don't do EV chargers, PV etc, so the following is really just guesswork. I'd have thought that if a circuit supplies power to the busbar via an RCD (e.g. solar PV via a bi-directional RCBO), then any fault to earth 'downstream' would cause said RCD to trip (by 'downstream', I mean the busbar and all other attached circuits).

 

[littlespark]

Let’s think of it another way.

Your PV during the day is generating power, sending it through the RCBO onto the busbar, and out onto other circuits.
As such, there is nothing coming from the grid, but may be going out to the grid….

In simple terms, you now have 2 rcbo’s “in series” and very close together. Either could trip if there’s a fault downstream on the non PV circuit.
This is repeated on every circuit.

If it’s the PV rcbo that is a little more sensitive, it will trip first, making you think the fault is there.


Perhaps the PV RCBO should be higher than 30mA, or time delayed. That way you would know which circuit had the fault as it would trip first.

Of course, we can’t do that, because regs would not allow.

 

[richy3333]

Morning.
I think that Proteus RCBO's have always been bidirectional - they just haven't shouted about it. Mention was made in a renewables podcast with them.

I think - and I'm happy to be corrected but if there is PV (not sure about and EV) if the PV is feeding back through into the CU any RCBO's could be affected as the PV will go through its own RCBO and could then pass through any other RCBO protection in the CU.

Under fault conditions of the PV circuit then the PV RCBO would trip (have seen this but not my installation!).

 

[Pretty Mouth]

If it’s the PV rcbo that is a little more sensitive, it will trip first, making you think the fault is there.

After the PV RCBO had tripped, isolating the PV supply to the busbar, the grid supply would take over wouldn't it? So I'd expect the RCBO protecting the faulty circuit to then trip also.

 

[littlespark]

I’m just thinking out loud really….

Ok, what about island mode, when the power is being taken from battery storage, and there is no grid?

I suppose, to us, it doesn’t matter. We might be called to find a fault, and normal testing would discover which circuit it was on.
But was just mulling it over one day in my head.

 

[UNG]

The debate of using bi-directional or uni-directional RCD / RCBO's will be driven by what the manufacturers choose to be their standard product in the, given that we have in some cases been fitting bi-directional RCD / RCBO's for a good number of years it will make little difference overall

 

[mainline]

Bidirectional rcds are more to do with the electronics not being damaged

I don’t think in the real world that it’s going to make any difference with tripping problems.

 

[happysteve]

The RCD part of RCBOs - and RCDs (RCCBs) for that matter - is only sensitive to an imbalance between the L and N current flowing through that device.

So say you have a fault on the (RCBO-protected) lighting circuit. Let's also assume, for the sake of argument, that your bi-directional RCBO on your PV circuit just happens to be a bit more spritely than your lighting RCBO. Let's further assume that it's sunny, and no energy is coming from the grid... your lights are being fed from the PV, through the PV RCBO, onto the L and N bus bars, to the lighting circuit.

Suddenly - disaster! - there is a short on a light between L and E...

Is the PV RCBO going to trip? It's a fair question. I could be wrong, but I don't think it will:

For the lighting RCBO, there is clearly an imbalance... the current goes through the L, but doesn't all return down the N, some at least instead going down the cpc to E. But let's assume it's sluggish or faulty, and slow to trip.

Just for ease of thinking about it, let's assume it's TN-C-S earthing arrangement, and the 2P main switch is closed (i.e. on). We're not "off grid", we're just not pulling anything from the grid, because it's sunny. The N and E are joined at the PME block / service head. This is effectively between the two RCBOs.

So from the point of view of the PV RCBO, the current will flow through the L, and it will return through the N. The fact that the current has taken a circuitous route makes no difference:

• L through the PV RCBO
• onto the L bus bar
• through the L bit of the lighting circuit RCBO
• through the fault onto the cpc
• back through the MET
• to the PME block / service head where the incoming L and N are joined together
• back through the N tail, through the N of the main switch
• on to the N bus bar
• through the N bit of the PV RCBO
• back to the source of energy (your PV battery or whatever)

So no imbalance between L and N on the PV RCBO.

Ah! But what if it's a TT system? Same argument, just an even more random route.


Happy to be schooled / laughed at if I'm wrong... it's probably the sort of thing John Ward could do a lovely explainer video on. Also, I don't know much about prosumer / battery / PV stuff, so no idea how this all fits into "island mode" or whatever, but my understanding is that - unless you're using electrical separation as your means of fault protection - there has to be a route between E and N for things to work.