additional RCD or not

[karter]

I would like to take power to a garden water feature from a spur off the conservatory ring circuit which is RCD protected via a switched FCU to be installed in the conservatory, the cable run being in 2.5 SY cable to the water feature. It has been suggested that it might be better to go via an RCD unit instead of a standard FCU, but in the event of a fault in the garden with both RCDs being 30mA would discrimination take place or would I be wasting money with a 2nd RCD.I am aware of building regulation part P and works will be inspected, tested and submitted. Please could you advise. Happy new year to all who participate in this great forum

 

[GaryMo]

If the ring final is already protected by a 30mA RCD then there's no need to use another 30mA RCD in the form of a FCU.
Just use a standard SFCU.

 

[ban-all-sheds]

2.5 SY cable to the water feature.

Where will the cable run? Is SY a suitable type? Will the water feature accept an SY gland?

but in the event of a fault in the garden with both RCDs being 30mA would discrimination take place or would I be wasting money with a 2nd RCD.

No and yes...

I am aware of building regulation part P and works will be inspected, tested and submitted.

Are you also aware that that's not the right order for doing things?

 

[karter]

Many thanks Gary mo and Ban for your speedy replies.Ban , I know from reading your replies how accurate and serious you take the advice you give. My electrician will be installing the SY and certifying to building control, it was just realy the 2nd RCD situation, could you please explain the "yes & no " part of your answer. many thanks again.

 

[Spark123]

Think he means no they wont disrciminate and yes you are wasting your money.

 

[NotHimAgain]

The two respondents are right in what they say if your only consideration is discrimination between the RCDs.

However, suppose you take the view that you really would like this RCD to work and save your life , if required - now you also need to consider reliability. RCDs can fail to work, the modes and rates of failure are complicated and sometimes disputed, but they can fail.

Now if we put two in series there is usually a square term improvement in failure rate for the combined set up. This is an over simplification, but it does give us an indication of the possible gain.

Well why should I make it simple .

 

[RF Lighting]

But RCDs are only there if every other method of protection has already failed, and someone ends up getting a shock.

You should never rely on an RCD as a device to prevent electric shock, and there is no need to design your installation as such.

There is absoloutly no point in having two RCDs in series*.









*Except of course in certain circumstances when using different RCDs for fire protection etc

 

[NotHimAgain]

There is absoloutly no point in having two RCDs in series*.

Well that is your point of view and you are entitled to it - but improving reliability by such methods is commonly used in many engineering systems.

 

[RF Lighting]

Do you add an extra layer of insulation to all your cables incase the first layer fails?

 

[lettselectrical]

well put NotHimAgain

 

[lettselectrical]

well put rf lighting

 

[NotHimAgain]

Do you add an extra layer of insulation to all your cables incase the first layer fails?

Yes its called variously - double insulation; class II; all insulated and reinforced insulation .

 

[RF Lighting]

That's not what I asked.

Do you add an extra layer of insultation to all your cables?

 

[NotHimAgain]

No but then cables are not electro-mechanical devices that have been shown by various studies to have a 'significant' (we could argue all night about that ) failure rate.

 

[RF Lighting]

Do cables ever present a hazard due to damage to their insulation?