Shower not on RCD

[wiggles]

Hi,

I’ve been reading through these forums and I’m hoping that some of the regulars can give me some advice please – and I know that you love pictures, so I’ve included some! I took these photos with my phone but I can take my digi cam over there tomorrow and take some more if these are not good enough – I’ll also take my tools and open the CU to get some shots with the wiring exposed (I will turn it off first).

I’m a little concerned about my parents CU setup. Their house and electrics are 25-30 years old. They had an extension built around 8 years ago which included the addition of an electric shower.

I looked at the CU today (it is a split load). The shower is on the non-RCD side of the unit. I’m concerned about this. The RCD (says earth leakage device - an older type of RCD or still ok?) side only seems to have room for two MCBs. Currently, the slots are used by downstairs and upstairs socket rings. I’m hoping that one of you is familiar with this model of CU and can advise me if:

The RCD can be moved along one space to the left to make room for the shower breaker.

OR

Is there a combined MCB/RCD (can’t remember what you call them) for this model of CU (JohnD will approve of this one)

OR

As a last resort, should I move the upstairs sockets to the non-RCD side to make room for the shower MCB.

OR

Something else?

Thanks.

 

[RF Lighting]

Unfortunatly AFAIK Crabtree no longer make any parts for that range of CU, and I don't remember ever seeing an RCBO for that CU.

We will get a better idea of wether the RCD can be moved once you have posted a clearer pic with the cover removed. I think the best idea might be to fit a seperate RCD in a small enclosure adjacent to the existing CU.

As it is, the shower is not immediatly dangerous, but as you know it is always best to have RCD protection for this sort of circuit.

 

[JohnD]

It's a Crabtree, I can't make out if it's an older model of Starbreaker or if it's Polestar or somethig. If you can see any part no's on the MCBs that should tell us. They might be numbers like 63/B20 or 610/20B

There probably are RCBOs to fit it, but they might not be readily available and they be be 2-module wide.

Did you take the pics with a phone? If you can take some with flash or bright light they may be clearer.

Is the local shower switch a ceiling-mounted drop-cord switch? You coud have a separate RCD near the CU or near the shower.

edited: bah, too slow

 

[wiggles]

Wow - quick replies.

Unfortunately I did use a phone and there’s no flash on it. I could only see how crap the pictures where when I got home. I will take my proper digi camera over tomorrow and get some decent photos.

There an isolation ceiling pullswitch next to the shower. It’s a while since I’ve seen it but I don’t remember it as being anything remarkable. Is there such a thing as an inline RCD (and more to the point, was there 8 years ago)? There are certainly no other CU enclosures anywhere else in the house.

 

[RF Lighting]

Jonh, it is an old starbreaker CU. Now you mention it, I have fitted a replacement RCBO to one of these CUs, but like you say it is 2 modules wide.

 

[Taylortwocities]

So, best solution is move upstairs ring to non-rcd side and pt shower on RCD side.

 

[wiggles]

As it is, the shower is not immediatly dangerous, but as you know it is always best to have RCD protection for this sort of circuit.

This is where I am a little fuzzy. My hobby is electronics so I have a good basic understanding theory wise but I’m trying to work out a scenario where it would be dangerous. I know an RCD looks for balance between phase and neutral and is therefore very sensitive to small current leakages. A good thing as it will quickly detect any flow from live to earth and open the circuit. I know the MCB is there to protect the cable and at 40 amps, it would take a significant overload to open. I guess what I’m trying to ask is in the absence of an RCD, under what circumstances would the shower become dangerous and what is already in place to protect the user?

 

[plugwash]

Having a RCD serves two functions

1: detecting leakages to earth before they become significant enough to cause dangerous voltage rises and/or dangerous heating.

2: detecting currrents leaking via a human body.

Essentially a RCD is an extra precuation for those times when the normal protections from electric shock and fire fail. Given that a shower is a very high power appliance in an area where people are generally wet and naked it makes sense to protect it. Also most manufacturers require it in thier instructions.

IMHO a rcd in a seperate 2 module box near the CU is the way to go.

 

[Adam_151]

Read the post here http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=7335 by mapj1

 

[wiggles]

Read the post here http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=7335 by mapj1

Great link - thanks. That was a really good scenario (worn seal) Now I can't understand why RCDs for showers are not mandatory

 

[JohnD]

I think TTC's suggestion is a very good one. It has the benefits of simplicity, speed and economy. It could be done in less time than we've spent thinking about it.

I would otherwise have gone for an RCD in a small enclosure; either adjacent to the CU, or else just outside the bathroom door or in the airing cupboard. If the shower had a wall-mounted DP switch (not a pull-cord) I would have been very keen to replace it with an RCD or possibly a very small CU with main switch as well (to reduce wear on the RCD).

 

[wiggles]

Having a RCD serves two functions

1: detecting leakages to earth before they become significant enough to cause dangerous voltage rises and/or dangerous heating.

2: detecting currrents leaking via a human body.

Not meaning to be pedantic here – just trying to increase my understanding and I’m grateful for the input so far.

I don’t understand your distinction between points 1 & 2.

Point 1 – I think I understand – you come into contact with live and provide a more attractive path to neutral via the earth. The current passing through you would be absent from the neutral part of the RCD circuit creating an imbalance.

Point 2 – I don’t understand this at all. Surely all the RCD can detect is an imbalance between the outgoing phase and returning neutral – could you please expand?

I’m trying to work through other scenarios that an RCD might detect – but I can’t think of any others other than phase to earth (with or without person in-between - nasty) or neutral to earth (which I don’t believe would be dangerous in the circumstances described).

 

[plugwash]

consider the following scenario, the heater can is a bit leaky. This is not immediately dangerous asuming things are earthed but may have the potential to especially if the earthing is poor. The rcd will detect this fault and trip before it becomes significant enough to pose a hazard. This is what i meant by my first point.

the second point is direct contact with a live point or with non earthed metal that somehow becomes live. In normal operation this shouldn't happen but things do get damaged. In this case the current flowing through the body should cause a trip hopefully before the current stops the heart.

 

[Paul Barker]

Wiggles since you know about electronics I'll put it in terms you understand.

There is a common ferrous core inside an rcd with four coils on it. The phase conductor is joined through one coil and the neutral through aother.

There are two further coils one of which operates a relay that cuts power and the other is attached to the test button though a current set resistor.

If there is an imbalance off current flowing between phase and neutral a magnetic field is created on the common core (usually torroidal but not significant), this powers the winding attached to the relay, which cuts power off in such a way that it has to be physically reset. The fourth (test button) coil speaks for itself, it also sends a defined current (controlled by the resistor) around the core which should cause the unit to trip, if it fails to do so the rcd is scrap.

Finally there are two possible ways by which the sense coil operates the relay which cuts the power. 1/ Mechanically via electromagnetism (solenoid) working with a spring mechanism which is just poised like a mouse trap waiting for a little tip over the edge to send it catapaulting into action. 2/ electronically via solid state amplification (which I would have thought was less optimal). But my interest in electronics stems from the era of thermionic valves so I tend to favour the old ways. To me mosfets and transistors are just fast blow fuses. Give me an electromechanical device any day of the weak.

Therefore in practice all that an rcd can do is sense an imbalance in current flowing through phase and neutral. If you draw too great a load legitimately (overcurrent) the rcd can do nothing about it. If you take hold of the phase conductor in one hand and the neutral conductor in the other the full prospective fault current of the supply could flow through you and the rcd would not take any notice. You would have to make contact with either the phase or the neutral only to cause an imbalance in the rcd.

It therefore does not offer sufficient protection alone, but it supplements the protection provided by the earthed equipotential bonding and the over current protective device (fuse or circuit breaker).

So for a safe shower, make sure that the enclosure is in good condition, that the place where it is situated is an equipotential zone which is at the same potential as the protective conductor of the shower, that it is protected using a sufficiently fast reacting suitably rated overcurrent protective device, and a residual current device.

 

[Adam_151]

The test button in 95% of cases connects a resister between the outgoing phase and the incomming neutral (or possibly vice versa) in order to create an inbalence, never heard of a separate winding for the test button