Three phase equipment RCD protection on a TT system

[IAmSparkytus!]

TT system, existing three phase fuse board recently installed by another spark which only has single phase circuits terminated into individual RCBOs

Required to install three phase circuits for new equipment, is there a better way of doing this than fitting a 4pole RCD ahead of the main switch?

Conscious that this is going to cause total loss of power if a problem ever occurs.

No space nearby to install a seperate DB for the three phase equipment only.

 

[Risteard]

TT system, existing three phase fuse board recently installed by another spark which only has single phase circuits terminated into individual RCBOs

Required to install three phase circuits for new equipment, is there a better way of doing this than fitting a 4pole RCD ahead of the main switch?

Conscious that this is going to cause total loss of power if a problem ever occurs.

No space nearby to install a seperate DB for the three phase equipment only.

Go from the triple pole circuit breaker into an adjacent DIN rail enclosure with your four pole RCCB.

I assume that there's upstream fault protection, e.g. an S-type RCCB?

 

[davesparks]

What make/model of DB?

Some have 3 phase RCBO or RCD conversion options

 

[IAmSparkytus!]

Go from the triple pole circuit breaker into an adjacent DIN rail enclosure with your four pole RCCB.

I assume that there's upstream fault protection, e.g. an S-type RCCB?

No room adjacent to the distribution board to be able to do this as there are 3 x three phase circuits to install so 3 x RCCBs and enclosures would be required.

No upstream RCD but didn't think this was a requirement of BS7671 if meter tails are visible and proper glands and termination are in place?

Easy enough to add a 100mA time delay RCD upstream of the board if required but would still need 30mA RCCBs for each three phase circuit.

Would an upstream 30mA 4P RCCB be ridiculous?

 

[IAmSparkytus!]

What make/model of DB?

Some have 3 phase RCBO or RCD conversion options

Lewden .. dont think they do an RCD conversion kit unfortunately!

 

[davesparks]

Would an upstream 30mA 4P RCCB be ridiculous?

Yes, that would be a non-compliant design.

 

[davesparks]

Easy enough to add a 100mA time delay RCD upstream of the board if required but would still need 30mA RCCBs for each three phase circuit.

What are the 30mA RCCBs required for?

 

[IAmSparkytus!]

What are the 30mA RCCBs required for?

My understanding is that all circuits on a TT system are required to be protected by a 30mA RCD .. is this wrong?

 

[davesparks]

My understanding is that all circuits on a TT system are required to be protected by a 30mA RCD .. is this wrong?

This is indeed wrong.

Installations with TT earthing often cannot guarantee a suitable Zs to be able to meet the requirements for fault protection.
So in order to achieve adequate fault protection RCDs are used.

If an RCD is required for fault protection it does not necessarily need to be 30mA and it may also be acceptable to have a time delay.

All other requirements for RCD protection, additional protection etc, still apply.

 

[pc1966]

What is the nature of the 3-phase circuit you are installing?

If it is for a socket outlet up to 32A or buried cables less than 50mm from wall surfaces then it would need 30mA "additional protection", but if not (above 32A sockets or hard-wired) then often not but obviously depends on what it is. Then the use of a something 100mA delay RCD up-front would allow fault protection, but with the risk of everything going off should that happen.