Max Zs on RCBO

[keefy]

Just tested property with a HUGE ring circuit and got 1.4 ohms on a type C RCBO. Given the max is .57 there is no chance of getting this close. Has anyone come across this before and what would be the advice given that it is protected by an RCBO and if you consider a TT system it is better than the usual readings you get there.I also know there has to be some consideration to ensure the device trips with a L - N fault!

 

[MarkieSparkie]

Hi Keefy, I assume 1.4 Ohms is the Zs for the Ring Final Circuit (RFC). In an RCBO (BSEN 61009-1) you effectively have an MCB and RCD in one device as I'm sure your aware, because the RFC has the addition protection of the RCD part of the RCBO your maximum Zs for the circuit is 1667 Ohms (Table 41.5 applies). So therefore, your RFC complies easily with BS 7671, no worries.
General domestic and resistive loads should be protected by a Type B device. Type C devices are usually required if there is an appreciable inrush current when a small motor (few kW), small transformer, bank of LF fluorescent lights or other types of inductive load are switched on.

 

[Deleted member 9648]

Just tested property with a HUGE ring circuit and got 1.4 ohms on a type C RCBO. Given the max is .57 there is no chance of getting this close. Has anyone come across this before and what would be the advice given that it is protected by an RCBO and if you consider a TT system it is better than the usual readings you get there.I also know there has to be some consideration to ensure the device trips with a L - N fault![/QUOTE]

433.1.1......433.1.2....435.1.
Where a protective device meets the requirements for overload protection and has a short circuit breaking capacity exceeding the value at it's point of installation it can be assumed to meet the requirements for short circuit protection.

 

[keefy]

Cheers you guys, excellent information.

Markie, its in a commercial building with very few appliances on and none that would give reason for a type C, but I have in the past found it hard to get hold of type B RCBO's!

 

[telectrix]

why not split the RFC int 2 radials and reduce the RCBO rating ( say from 32A to 16A )

 

[keefy]

why not split the RFC int 2 radials and reduce the RCBO rating ( say from 32A to 16A )

Nice idea.. no chance given the cable route is a massive run all the way around the building which will result in 2 legs being installed at not much less length than the existing cables. Like I said, nice idea... but there is always a reason why someone could not be arsed to do a good job in the first place!

 

[Guffer]

Nice idea.. no chance given the cable route is a massive run all the way around the building which will result in 2 legs being installed at not much less length than the existing cables. Like I said, nice idea... but there is always a reason why someone could not be arsed to do a good job in the first place!

Why bother to split the ring ? If the circuit is not intended for heavy load just replace with the 16A RCBO anyway you get the benefit of the higher Zs and everything still complies ?

 

[Deleted member 9648]

Why bother to split the ring ? If the circuit is not intended for heavy load just replace with the 16A RCBO anyway you get the benefit of the higher Zs and everything still complies ?

Hopefully he got this sorted last october.

 

[Paul P]

Just tested property with a HUGE ring circuit and got 1.4 ohms on a type C RCBO. Given the max is .57 there is no chance of getting this close. Has anyone come across this before and what would be the advice given that it is protected by an RCBO and if you consider a TT system it is better than the usual readings you get there.I also know there has to be some consideration to ensure the device trips with a L - N fault![/QUOTE]

Regarding the L-N fault tripping, doen't the low Zs value corispond to the disconection time required for an earth fault thus keeping the potection shock voltage to earth low. As a L-N fault does not need to be disconected within 0.4 of a second can you not rely on the RCD value of 1667 ohms. Also on a 2.5 ring the L-N reading would be higher due to both L & N being 2.5mm.

 

[Deleted member 9648]

[/QUOTE]

Regarding the L-N fault tripping, doen't the low Zs value corispond to the disconection time required for an earth fault thus keeping the potection shock voltage to earth low. As a L-N fault does not need to be disconected within 0.4 of a second can you not rely on the RCD value of 1667 ohms. Also on a 2.5 ring the L-N reading would be higher due to both L & N being 2.5mm.

See post 2!