Distribution circuits in Section 705 location, RCD needed?

[timhoward]

I just want to check how I should be interpreting section 705.

705.411.1 states required disconnection devices, the first two indents covering final circuits and sockets, requiring 30ma and 100ma RCD's depending on rating of socket.
Indent 3 says " all other circuits" need 300ma RCD.

Does that include distribution circuits, outside? Or is the word "final" implied?

The background - a fused down main supply circuit runs along the drive of an estate. About half way along the drive there is a main distribution cabinet. This used to be the drive side of the fence but has been moved to other side of fence, field of (usually) sheep.

It's in a massive IP rated double door enclosure. Several distribution circuits leave here going underground, with about 1 ft of SWA being accessible from ground to bottom of enclosure.

2 new circuits run to new hook up points within the same field. (This has created multiple issues with isolators and wiring systems being accessible to animals, and these will need to be resolved)

But the main question is whether an distribution circuit with the source and destination within a field with livestock needs to comply with 705.411.1 iii)
A secondary question is whether an underground distribution circuit with the source in a field with livestock and the destination elsewhere e.g. a house needs to comply with 705.411.1 iii)

It feels as though the intent of that reg is fire protection and aimed at indoor circuits but the wording is actually quite vague.
Getting the cabinet fenced off (carefully missing cables!) is of course an option.

 

[7029 dave]

Whats the earthing arrangement.

 

[timhoward]

Whats the earthing arrangement.

Mainly TNCS.
One set of hook-up points is definitely TT'd.
I'd need to check the other but I believe it is much higher and connected to supplier earth.

I'd also comment that Section 705 didn't say quite what I thought it said about that.
It says it shall not be TNC (combined and never separated again) with a note underneath saying that it doesn't preclude the use of TNCS. (TNC is banned in the UK anyway making that bit somewhat of a red herring)

I'd also comment that we are talking about 2 fields here, no farm buildings containing livestock at all.
The more I look at this place the more complex it gets. A diagram is forthcoming....

 

[7029 dave]

It's certainly very confusing, reading through it, my take is this distribution circuit needs at least 300 Ma RCD protection, due to the location. Also it will give selectivity and discrimination for RCD protection used at the other end of the circuit/ circuits. What's your opinion Tim

 

[timhoward]

Thanks for looking through it.
My take is that it's a circuit in a location covered by 705 and needs RCD protection.
I still believe the intent of the reg is fire protection in buildings, not distro circuits buried underground.

This distribution box is a massive Hager board with x160 moulded case circuit breakers. It would have cost a fortune.
Not enough thought went into moving it to it's current location.

 

[nicebutdim]

Am I reading the above correctly - a single panel board with 160 MCCBs?

 

[timhoward]

Am I reading the above correctly - a single panel board with 160 MCCBs?

Not quite
6 circuits, similar to this:

MCCB h3 x160 TM 3x100A 18kA • HDA100U | Hager

Find products, Moulded Case Circuit Breaker h3 x160 TM ADJ 3P3D 100A 18kA CTC and more on Hager United Kingdom

hager.com

 

[nicebutdim]

Not quite
6 circuits, similar to this:

MCCB h3 x160 TM 3x100A 18kA • HDA100U | Hager

Find products, Moulded Case Circuit Breaker h3 x160 TM ADJ 3P3D 100A 18kA CTC and more on Hager United Kingdom

hager.com

I was struggling to comprehend the idea of a panel board with 160 distribution circuits

 

[mainline]

705.411.1 states required disconnection devices, the
Indent 3 says " all other circuits" need 300ma RCD.

(iii) In all other circuits, RCDs with a rated residual operating current not exceeding 300 mA.

Is this just a distribution cabinet with no outlets


705.513.2 Accessibility by livestock
Electrical equipment generally shall be inaccessible to livestock.

 

[timhoward]

Is this just a distribution cabinet with no outlets

Yes.
Quick sketch:

Distro cabinet D has no outlets, it's a metal distribution panel within an IP rated box mounted within a timber 'bus shelter' style structure.
Enduring question is whether D being inside the field mean outgoing distribution circuits need 300ma RCD protection.

Other concerns exist about Sockets A being low and accessible to animals. They are at least all RCD protected and TT'd.
Rotary Isolators are also accessible to animals which is specifically mentioned in another reg.
So there are definite issues there.

Sockets B are much higher up, well out of reach of sheep, but are 2 x 32A sockets and 1 x 63 amp, and they are all 300ma integral RCDs, so that is another problem.

705.513.2 Accessibility by livestock
Electrical equipment generally shall be inaccessible to livestock.

I believe the panel within an IP rated cabinet is sufficient protection.

The pragmatic side of me says that if the cable is deep enough, the only difference between it being there and not being there are the short sections between ground and panel, and if they were adequately mechanically protected then I can't see any benefit to RCD protecting it. As I've said before, I think fire protection inside a building is the intent of the reg.

 

[brianmoooore]

Sheep won't hurt anything. Cattle won't bite anything, but they'll lick and pull with their rasp like tongues. Horses are the ones you need to worry about.
Reminded me of the SWA I left in an open trench overnight. Horse found it. Pity it wasn't energised.

 

[timhoward]

Sheep won't hurt anything. Cattle won't bite anything, but they'll lick and pull with their rasp like tongues. Horses are the ones you need to worry about.
Reminded me of the SWA I left in an open trench overnight. Horse found it. Pity it wasn't energised.

...and that is the thing. I don't think the RCD bit is actually a problem in the real world at all.

I want a C3.5 - "Improvement NOT Recommended but I've noticed it"

 

[westward10]

Section 705 does not differentiate between final circuits a distribution circuits so in my opinion 300ma RCD protection is required.
As you say 300ma is often used for protection against the risk of fire, see Regulation 532.2 and 705.422.7.
I agree that this must be an extremely low risk in terms of a fire hazard but I'm not sure how you can interpret it any different.

 

[timhoward]

@westward10 I really appreciate your thoughts. Thanks.

 

[loz2754]

For what it's worth I agree with Westward.

Hence - all circuits, whether final or distribution, must be protected by 300mA RCD at the top of the tree. Then final circuits that feed socket outlets need either 100mA or 30mA, depending on the rating of the socket outlets. This applies whether the location is indoors or outdoors, according to the scope at the beginning of section 705.

It may also apply to other locations that are in common buildings, for example - the cable that feeds the location may also need to be protected by a 300mA RCD.

As to the reason for the 300mA RCD being required, it is not for fire protection, as this is dealt with as a separate requirement in 705.422.7