TT and RCD Protection

[thomas-1981]

I was doing work on an commercial site today. The supply is TT and the install is about 25 years old. I was under the impression a TT system had to be RCD protected to meet disconnection times. But this install only had a few circuits with RCBO's. Would the version of regs the install was designed to not require RCD protection? Thanks

 

[telectrix]

around that time it was usual to fit an up front 100mA RCD on a TT system. if you are doing any work in it, i'd be inclined to fit a new CU with 100mA delay RCD as main switch and 30mA RCD/RCBOs for all circuits.

 

[spark 68]

I was doing work on an commercial site today. The supply is TT and the install is about 25 years old. I was under the impression a TT system had to be RCD protected to meet disconnection times. But this install only had a few circuits with RCBO's. Would the version of regs the install was designed to not require RCD protection? Thanks

If it had RCBO,s this would give you your RCD protection would it not ?, unless you meant MCBs ?

 

[MDJ]

He did say a few circuits which probably means not all is protected guys

 

[spark 68]

Ah! right MDJ,

I read it as there were only a few circuits, (all) with RCBO protection, cheers,

In that case ^^^ what Tel said

 

[happysteve]

"Shall we eat grandma?"
"Shall we eat, grandma?"

Big difference.

One day... One day, I will have summat useful to contribute other than inanities.

 

[D Skelton]

I was doing work on an commercial site today. The supply is TT and the install is about 25 years old. I was under the impression a TT system had to be RCD protected to meet disconnection times. But this install only had a few circuits with RCBO's. Would the version of regs the install was designed to not require RCD protection? Thanks

What was the Ra?

 

[richy3333]

"Shall we eat grandma?"
"Shall we eat, grandma?"

Big difference.

One day... One day, I will have summat useful to contribute other than inanities.

Its been 22 minutes since your post. Did you eat Grandma??

 

[happysteve]

Its been 22 minutes since your post. Did you eat Grandma??

Om nom nom...

 

[HandySparks]

What was the Ra?

Just the question I was thinking of.

If the earth fault loop impedance is low enough, TT without RCDs (where not required for other reasons) may be OK.

 

[onions1066]

Now your interpretation of eating grandma was different to mine :ack2:

 

[PEG]

What was the Ra?

Ra? Ra? ......i've worked in Liverpool where that's all they ever asked....

 

[Engineer54]

What was the Ra?

A very important question, when considering this is an industrial installation.

Still amazes me just how little many sparks know about TT systems, especially when talking about industrial and medium to large commercial applications....