Wet Locations

[Cookie]

In wet locations a body resistance of 200 ohms is assumed, and disconnection times go down to 0.2 seconds in a 230 volts supply (0.4 110 volt supply), is that correct? Or is this not required in BS7671?

 

[pc1966]

I agree. I,m also a little perplexed as the UK generally tends towards good order and pragmatism

The problem is society is not uniform in attitudes, education, and beliefs. While the UK is reasonably safety conscious in many ways, for example one of the lowest road death rates in Europe, we also have quite a capacity for stupidity as well.

All regulations are a trade off between convenience, cost, and safety. I guess the IET/BSI have generally taken the view that they ought to set standards for electrical safety that are designed to reduce the impact of the more Darwin-inclined members of society.

Some things like hair-dryers are often seen in hotels, but hard-wired in to FCU and not within reach of bath/shower. Having sockets is more of an open-season on more survival-challenged members of our nation.

 

[brianmoooore]

701:555.05. "where a prefabricated shower cabinet is installed in a room other than a bath or shower room, any wall switch and any socket outlet shall be at a distance of at least 0.6 mts from the door opening of the shower or cabinet"
This reg is almost certainly "copied and pasted" from the UK. Its intention was to facilitate the likes of budget hotels that don't have the space to conform to the regs. It allows contractors dealing with unconventional domestic installs to assist customers with creative solutions.

In the UK regs., 15th edition (1981) states NO sockets in bath/shower rooms, but does refer to rooms other than a bathroom, where it says 2.5m min.
16th edition (Amendment 3, 2000, I think) introduces the zone concept for bathrooms, saying no sockets in zones 1,2 and 3, which add up to 3m, so sockets now allowed as long as 3m+ away.
There's a reference to other rooms, but it's still 3m+, and sockets over that distance must be RCD protected.
17th edition (2008); zone 3 has gone, but it still says 3m+ for a room with a bath or shower, with no reference to other rooms at all that I know of, so still 3m+
The advance of time spared me the 18th in detail, but AFAIK, there's no change.

 

[pc1966]

This subject appeared in another thread a bit back about needing a socket in a bathroom for a helper to a disabled person. Some discussion went around the idea of using an IP65 rated socket (as allowed outside in the rain) just out of zone 1, etc, but nothing really satisfactory was reached.

In that case probably a written risk-assessment and justification would be the installers get-out: where the disabled person's need for the power-assisted help and the professional nature and understanding of those involved would be adequate justification for departing from the regs.

But for many it would be far better if certain exemptions and/or guidance was available to deal with the risk of doing so.

 

[LastManOnline]

Rain vs wet hands is a bit different, but, at the same time, rain water has a resistance of 20,000 ohms.

Out of curiosity. 20,000 ohms in what scenario.? Would that be for instance a meter of 20mm plastic pipe full of rainwater, measured end to end?

 

[Avo Mk8]

I think it's that rainwater has a resistivity of 20k ohm cm. (figures vary!)
formula on the web!
I make it about 32 Megohms for the metre of plastic pipe, but no doubt I've made an error there somewhere!

Electrical Resistivity - Definition, SI Unit - Teachoo - Concepts

Resistivity is the characteristic property of the material by which it resists the amount of current through it.What is the SI Unit of resistivity?SI Unit of resistivity is ohm meter.Explanation.We know that,R = ρ (l/A)Therefore,ρ = R (A/l)= ohm × m2/m= ohm × mTherefore, unit of rho is ohm meter.Not

www.teachoo.com

 

[pc1966]

I think it's that rainwater has a resistivity of 20k ohm cm. (figures vary!)

Interestingly it seems tap water is far more conductive at 1000-5000 ohm.m from here:

Understanding the Resistivity of Water - Sensorex Liquid Analysis Technology

Understanding the resistivity of water is important for a number of reasons. This article discusses this and how you are able to measure it!

sensorex.com

Down to 200 ohm.m for brackish river water. Not sure what a bath filed with soapy stuff would be!

 

[Cookie]

Interestingly it seems tap water is far more conductive at 1000-5000 ohm.m from here:

Understanding the Resistivity of Water - Sensorex Liquid Analysis Technology

Understanding the resistivity of water is important for a number of reasons. This article discusses this and how you are able to measure it!

sensorex.com

Down to 200 ohm.m for brackish river water. Not sure what a bath filed with soapy stuff would be!

In theory the voltage gradient would be worse in fresh water than salt water.

 

[Cookie]

Alright, so I came across this:

Supplementary Bonding in Bathrooms: 17th Edition Wiring Regs

Understanding supplementary bonding in bathrooms and shower rooms under 17th Edition Wiring Regulations. Learn when it's required and when it can be omitted.

studylib.net

I am thinking that if disconnection time can be reduced to 0.17 seconds, the RCD is not needed from a safety sense. It would be nice is BS7671 took this into consideration.

Just thinking out loud.

 

[pc1966]

I am thinking that if disconnection time can be reduced to 0.17 seconds, the RCD is not needed from a safety sense. It would be nice is BS7671 took this into consideration.

No, the RCD is still needed in my view unless you have eliminated single-fault risks.

So if you have supplementary bonding then you have to have two "unobserved" faults before a short L-E becomes dangerous, both the original CPC and the supplementary bonding, to expose a person to a dangerous voltage. Up to the 16th (I think) that was the norm for wet areas in the UK.

If you only had the CPC and very fast OCPD then an open CPC fault leaves the person completely unprotected against a L-E fault.

Now that RCDs are cheap and readily available they should be used for all sorts of reasons. I still don't like depending on them as my primary means of disconnection due to the single "unobserved" fault risk (and higher probability of that than MCB sticking or fuse somehow failing to blow), but on any new systems they make sense as additional protection.

 

[Cookie]

No, the RCD is still needed in my view unless you have eliminated single-fault risks.

So if you have supplementary bonding then you have to have two "unobserved" faults before a short L-E becomes dangerous, both the original CPC and the supplementary bonding, to expose a person to a dangerous voltage. Up to the 16th (I think) that was the norm for wet areas in the UK.

If you only had the CPC and very fast OCPD then an open CPC fault leaves the person completely unprotected against a L-E fault.

Now that RCDs are cheap and readily available they should be used for all sorts of reasons. I still don't like depending on them as my primary means of disconnection due to the single "unobserved" fault risk (and higher probability of that than MCB sticking or fuse somehow failing to blow), but on any new systems they make sense as additional protection.

Correct, but as you say I do not like an RCD being the primary means to protect a user. In my opinion they supplemental or backup protection.

 

[LastManOnline]

Correct, but as you say I do not like an RCD being the primary means to protect a user. In my opinion they supplemental or backup protection.

Agreed. An rcd as a primary means of protection never sat right. It's best suited to a "support role", basically plugging the holes that primary protection can't provide. It plays a supplementary but vital role.

 

[Cookie]

Agreed. An rcd as a primary means of protection never sat right. It's best suited to a "support role", basically plugging the holes that primary protection can't provide. It plays a supplementary but vital role.

Agreed.