Short Circuit Currents of RCCB & RCBO

You are using the terms RCCB and RCBO, which implies that you already know the difference between them. RCCB does not have overcurrent capacity, while RCBO has.

Please check out this link : http://en.wikipedia.org/wiki/Residual-current_device

The choice of what short-circuit level you need should be based on a good study. In case you can't find this figure, you need to pick the best that is available.

Good luck.

This begs the question, do your employers allow you to specify purchasing requirements?

From some of your recent posts on the board you don't seem able to make a decision.

Thank you, TonyS.

Before you attempt to make any decisions, you should perform good short-circuit study in your system and them check the products (manufacturer and other info available); only them you will know what do you really need. If you make your decisions only based on the consultant, you might make a big mistake. Usually, consultants offer what will bring them more profit or easier work!!!!!! Well, with today's economy, who wouldn't do that?

"Between 4.5kA & 6kA- you have to select the breaker whose breaking capacity is more than the fault level at the point of installation. If the fault level is less than 4.5kA, then there is no point in going for a 6kA MCB. May be the consultnt wants tio be safe as he may not be aware of the fault level at your installtion. So, he suggests 6kA". I think this is a good piece of work.

But, it is not correct when you said "RCCB does not have SCprotction capability. Only an RCBO has". RCBO has over current protection only.

Moreover, is there any confusion between your Short circuit and over current protection? I think we just can apply delay for over current protection only. It is too dangerous when we intend to delay in short circuit protection.

As my conclusion, the SCs rated on those RCCB and RCBO are withstanding capability. The bigger they are, the better they can withstand.

hien.nguyenquoc,

You have some misconceptions which may prove lethal. Please check out elsewhere, if you do not believe EE65 and me and others here.

1. MCBs, RCCBs and RCBOs DO NOT have SC withstand capacity, they are current limiting devices. Yes, the big devices have withstand capacity, I_cw. See Air Circuit Breakers.

2. RCCBs DO NOT have the capacity to break short-circuit currents. They need other SCPDs in addition, as mentioned by EE65. When the two devices are combined into one, it is called an RCBO.

3. "Overcurrent" is a generic name given to overloads and short-circuits. An RCCB has no overcurrent protection at all, it is only for earth leakage protection.

The devices you are referring to are to protect people in a home or an office. Please be very careful.

1. If so, what does the SC rating on the RCCB and RCBO means? Does it only imply that these equipments should be used if the pre-calculated SC in the system is less than that?

2. RCCBs DO NOT have the capacity to break short-circuit currents. I do not have any doubt in this. I am having in my hand the catalog (very famous manufacturer in EU, sorry I will not show the name not to damage that brand name) which written that: "RCBO - Against overcurrent and insulation fault". So I think that it will be much better and safer if they write "RCBO-Against Short circuit and insulation fault". This is the point making us confused.

3. Overcurrent and SC cannot be used interchangeable. But in the EE65 implied that we can delay the SC interrupting time? Is that correct? I think it is unsafe except for the case of overcurrent.

That is why I am struggling to find the hidden truths in these things. That is why I am questioning you until it is very very clear. I will allow them to use it.

_{RCCBs DO NOT have the capacity to break short-circuit currents. I do not have any doubt in this. I am having in my hand the catalog (very famous manufacturer in EU, sorry I will not show the name not to damage that brand name) which written that: "RCBO - Against overcurrent and insulation fault". So I think that it will be much better and safer if they write "RCBO-Against Short circuit and insulation fault". This is the point making us confused}

i already said "Overcurrent" includes overload and short-circuit. So, nothing wrong with what the manufacturer says. And he is saying that about RCBO. Not RCCB.

_{Overcurrent and SC cannot be used interchangeable. But in the EE65 implied that we can delay the SC interrupting time? Is that correct? I think it is unsafe except for the case of overcurrent.}

SC is a subset of overcurrent. Of course we can delay the tripping of an upstream circuit breaker to allow a downstream one to clear the short-circuit. EE65 is quite correct. The whole system of time-based discrimination in distribution systems is based on this. But remember, these generally are ACBs, which have the ability to withstand the short-circuit current for some specific time, usually 1 second. Check the web for discrimination systems, there is a lot of information out there.

EE65:

Thank you for correcting me. I got a mistake in interpreting RCCB and RCBO and am really sorry for this confusion.

RCCB does not have SC protection.

RCBO is a combination of RCCB and SCPD.

Normally, I use one RCCB combined with MCB to provide full protection for the downstream circuit. The problem looks better now and I think I will go on with that.

EE65,

Could I change the topic a bit more to the breakers?

So, to have a good discrimination in power system, we need to choose the correct breakers. As you mentioned the SC capacity should have a withstand time, when refer to this below photo, I could not find it.

This is leading me to the below idea.

We will select the downstream breakers which have a little faster tripping time. But the breaker itself is not enough. We need to check its serial number, then refer to the catalog or manual to check out the protection curve/SC withstand time. The lower curve will show the downstream breaker, the higher curve will show the upstream breakers? Is my understanding same as yours?

Depending on the regulations where you are based, an RCCB may or may not need to have it's short-circuit capacity printed on the case but regardless of this it will very definitely have a short-circuit capacity that the manufacturer has had it tested to.

The trick here is to download for yourself a copy of the manufacturer's datasheet or technical catalogue, and have a good look in there. In the case of the MG brand in your photo, it should be child's play to find this data.