DP RCBOs

JohnD said:
Is it overcurrent protection on the N?

Which is not normally provided anyway.
Click to expand...

If there is over current on the Neutral that is coming from a Live to Neutral path ( fault or overload ) then the over current on the Live will trip the MCB mechanism.

If there is over current on the Neutral that is NOT from a Live to Neutral path then the difference in Live and Neutral currents will trip the RCD mechanism.

That assumes that the RCD sensor will react to a very high difference in Live and Neutral current. A very large difference may saturate the sensor in the RCD. The very high difference would normally be Live current greater than Neutral current and hence the MCB would operate
 
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JohnD said:
What does a DP RCBO do that an SPSN doesn't?

Is it overcurrent protection on the N?
Click to expand...
Yes.

A double pole RCBO will trip on overcurrent in either the line or neutral.
A SPSN RCBO only trips on overload in the line, but will disconnect the neutral and line.

Same principle as a double pole MCB which will trip on overcurrent in either conductor
or a single pole MCB with an auxiliary contact attached, which only trips on overcurrent on one pole, but can disconnect something else at the same time.
 
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flameport said:
Yes.
  1. A double pole RCBO will trip on overcurrent in either the line or neutral.
  2. A SPSN RCBO only trips on overload in the line, but will disconnect the neutral and line.
Click to expand...
thx. So SPSN is not a full DP, but better than a SP RCBO. A 1.5 affair.
After all these posts.
 
But surely it will trip on a N overload, in one of the two ways Bernard says.
 
It probably would, but RCDs are not designed to protect against overload or short circuit, and there are a few situations where both poles would need overcurrent protection.

In a caravan where the possibility of the connected supply having L&N reversed being one of them.
This is correct:
bernardgreen said:
That assumes that the RCD sensor will react to a very high difference in Live and Neutral current.
Click to expand...
and there is also the possibility of the RCD part failing, particularly as they are all electronic now, which in the case of a caravan with reversed L&N supply would leave no protection against a fault from 'N' (actually L) to E.

Another situation would be a split supply with two lines and no neutral.
 
JohnD said:
What does a DP RCBO do that an SPSN doesn't? Is it overcurrent protection on the N?
Click to expand...
I believe that is the case, but ...
JohnD said:
Which is not normally provided anyway.
Click to expand...
Quite so.

I think one really has to do some 'barrel scarping' to think up (exceedingly improbable/rare) scenarios in which there would/could be over-current in a neutral but not the corresponding L. Indeed, the 'over-current' would presumably have to be coming from the L of some circuit, so presumably some (L-only-sensing) OPD would clear the fault.

Kind Regards, John
 
SBS trade Sales (other makes & suppliers are available) do compact RCBOs which use a neutral bus bar rather than flying leads - so a much neater board. Also SPSN, and the range is switching to type A sensing so suitable for pulsing DC loads.
If you want an MCB only circuit, he has half-width MCBs and a connector for the neutral cable to the bus bar.
 
JohnW2 said:
I think one really has to do some 'barrel scarping' to think up (exceedingly improbable/rare) scenarios in which there would/could be over-current in a neutral but not the corresponding L.
Click to expand...
As evidenced by the lack of "OMG, these new CUs we're fitting don't have OC protection on the neutrals" discussions :whistle:
 
SimonH2 said:
SBS trade Sales (other makes & suppliers are available) do compact RCBOs which use a neutral bus bar rather than flying leads - so a much neater board. Also SPSN, and the range is switching to type A sensing so suitable for pulsing DC loads.
If you want an MCB only circuit, he has half-width MCBs and a connector for the neutral cable to the bus bar.
Click to expand...
Neutral bus bar for RCBs. Sounds neat. Sounds so obvious.
 
Yes it's neat, yes it's obvious :whistle: And several (smaller) manufacturers do them.
But from the PoV of most manufacturers, they have an investment (both in tooling for what they sell, and installed base they need to support) in single bus-bar CUs. Basically there's zero compatibility between "traditional" CUs and dual-bus-bar CUs like the SBS ones so to introduce these dual-bus-bar devices means a new range of CUs, with all the tooling, stocking, distribution, and marketing costs - all while trying to persuade the industry that flying neutral leads are a PITA.
Being small is definitely an advantage. SBS will do you a custom arrangement - so if you want a few MCB ways, you have the option of custom bus-bars with a few ways that only fit single pole MCBs while the rest of the board takes the double pole devices. Or you can use the 1/2 pole MCBs which he only has in a limited range of ratings.
If you look carefully here, you can see the ends of the bars - the tabs are bend at 90˚ so that the bars sit flat along the bottom of the devices, one behind the other.
20190516_144630.jpg
 
D

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