Confusion on earth potential and continuity in bathrooms

[GMjohn]

Hi, I'm new to the forums so am not sure if this is the right section for these questions, but it appears to be a good area to place them.


I'm pretty new to learning about electrical installation, and as i go along, i realise there's still some gaps in my understanding which I can't seem to find clear satisfactory answers in the regs, text books or on the internet. So I thought I'd join up and ask what many experienced electricians will probably consider obvious and stupid little questions, but I'll never develop the deeper understanding of things that I want if I don't ask. Maybe it will also help other learners in the same boat as me.


I find the subject of earthing and bonding in bathrooms to be a source of confusion. I can't fully understand the actual situation with bathrooms and earth potential, and what the underlying principles are behind it.


I understand about it being a higher shock hazard due to the bare feet and wet conditions lowering bodily resistance, but don't have a good grasp of how this affects the earthing issue and some of the advice I have seen.


To illustrate, in the last few months I've come across quite a few installations where the wiring has been different.


For example, a bathroom with 4 230v light fittings where the supply went into a junction box and all the cables radiating out of this to the fittings had the cpc cut off at both ends so each cable had no cpc continuity. Is this correct? Wouldn't it have been better to have had cpc continuity down every cable going to the fittings, and just not terminate the cpc at the fitting, or is it normal and to regulation for these cables in the bathroom ceiling to not have cpc continuity down them?


I guess I'm asking is it correct to cut off the cpc at the bathroom entrance and have no cpc continuity going through any cable run in the ceiling to bathroom fittings, or should there be cpc continuity in these cables going around the bathroom ceiling even if there is to be no termination at the actual fitting?


Same when dealing with low voltage downlights. Every installation i've seen had a junction box at each fitting which fed a single transformer for each light. The cpc was not taken down to the light fitting on any installation, but on one installation I've seen the cpc cut off at both ends of all the cables so there's no earth continuity at all between the cables running from junction box to junction box in the ceiling. In other installations I've seen the cpc terminated at each junction box so the cables in the bathroom ceiling do have continuity. Which is the correct way, or is either acceptable?


I came across a bathroom the other week which had a 230v wall and ceiling light, both with exposed conductive parts which had earth continuity when tested. The ceiling light fitting had a single cpc conductor being run separately to a junction box in the loft as the light fitting wiring only had line and neutral conductors. Is this correct, or did the person who installed it do something wrong by introducing this earth continuity into the bathroom fittings?


I see it mentioned about not introducing earth potential into bathrooms, yet also see supplementary bonding being a necessity under certain conditions, which then introduces earth potential into these areas. I just don't fully understand the underlying principles behind these rules. If its better to keep earth potential out of bathrooms, why have this bonding?


If it's ok and safe to keep earth potential out of bathrooms, why allow earth potential into other areas of the house?


I'm obviously getting mixed up on this subject. I understand the general principle of equipotential bonding and earthing ensuring that exposed and extraneous conductive parts remain at the same potential so no current will flow if two different parts are touched. But if it is safer for that reason, why shouldn't exposed conductive parts in bathrooms be earthed, yet extraneous conductive parts should be? Won't they be at different potentials? Is it something to do with water spray and conductivity?


As you can see I am a little confused. No amount of reading or sources I find give me a satisfactory understanding. Can anyone help by answering these questions or pointing me to a good resource on the subject? I appreciate these may sound like stupid questions, but my lack of clarity on the matter has been bothering me for a while.

 

[Geoffsd]

You know it's not right to cut CPCs otherwise you wouldn't be asking.
They should NEVER be cut.
They should be connected at the source and terminated safely at the load if not required.

I came across a bathroom the other week which had a 230v wall and ceiling light, both with exposed conductive parts which had earth continuity when tested. The ceiling light fitting had a single cpc conductor being run separately to a junction box in the loft as the light fitting wiring only had line and neutral conductors. Is this correct, or did the person who installed it do something wrong by introducing this earth continuity into the bathroom fittings?

Of course exposed conductive parts should be earthed (unless it is a class 2 fitting)

I see it mentioned about not introducing earth potential into bathrooms, yet also see supplementary bonding being a necessity under certain conditions, which then introduces earth potential into these areas.

Wrong way round.
They are bonded because they are liable to introduce a potential - generally earth potential.
If they do not then they do not require bonding.
It is not yes or no depending on what the part is.

I just don't fully understand the underlying principles behind these rules. If its better to keep earth potential out of bathrooms, why have this bonding?

It would be better to keep it out - but we can't.

If it's ok and safe to keep earth potential out of bathrooms, why allow earth potential into other areas of the house?

We can't keep it out of bathrooms. Bonding equalises it.

I'm obviously getting mixed up on this subject. I understand the general principle of equipotential bonding and earthing ensuring that exposed and extraneous conductive parts remain at the same potential so no current will flow if two different parts are touched.

Yes.

But if it is safer for that reason, why shouldn't exposed conductive parts in bathrooms be earthed,

They must be - so the OPD operates in a fault situation,

yet extraneous conductive parts should be?

Not earthed, they are bonded because they are earthed somewhere else - so that potential difference is equalised.

Won't they be at different potentials?

Not if bonded.

Is it something to do with water spray and conductivity?

As you said, it is more dangerous when the person is standing in water and obviously it has to do with conductivity.

As you can see I am a little confused. No amount of reading or sources I find give me a satisfactory understanding. Can anyone help by answering these questions or pointing me to a good resource on the subject? I appreciate these may sound like stupid questions, but my lack of clarity on the matter has been bothering me for a while.

Google 'bonding and earthing'.

 

[Outspoken]

I think you need to understand what electrical potential is, if you understand that then you understand the overarching principle behind earthing in any location, not just bathrooms.

 

[Richard Burns]

Everyone has answered but I have written this so I am going to post it!

Difficult on a forum to give a full explanation of earthing and bonding, touch voltages and disconnection times.
However Guidance note 8 : Earthing and bonding would be well worth purchasing and can give some good advice to help you to understand some of the fundamental principles.

As a brief overview without going into too much detail (perhaps) lets cover some of the basics first:
In general it is not practical to insulate an entire building from earth so there are always earth potentials present. There is a specialised method of earth free local equipotential bonding that requires specific design to eliminate earth potential but this is not easily achievable on a large scale.
Therefore most electrical installation in general use are designed to be safe to use with these earth potentials present.

Earthing:

For class II equipment (fully insulated so that there is no possibility of any live part being exposed in case of a fault), SELV (separated extra low voltage that is not dangerous in terms of electric shock as it is not connected to earth and the voltage is below sensation level) and electrically separated supplies (voltage that has no reference to earth):
It may not be necessary to run a cpc into the accessory, however the cpc should be run up to the accessory. It may be there is metal there as well but this must not be earthed as it cannot become live (or present an earth potential) by the nature of the construction of the equipment.

Other than that there are requirements for protective earthing that mean that a circuit protective conductor must be run to every point in the wiring and to each accessory. This is to provide an earth path for fault current to flow and permit automatic disconnection of the supply.

Disconnection of the supply automatically in the case of a fault prevents persons or live stock receiving a shock, should they touch anything that would otherwise be live. The speed of disconnection should be fast enough to prevent a fatality.

Bonding

However despite the fast disconnection provided by ADS, there is still a period of time during the fault when the exposed conductive parts become live (and if the ADS does not occur for some other reason they will remain live).

At this point if someone were to touch the live parts they would likely receive a shock (even if only for 0.4s). The electric shock occurs because the person touching the live exposed conductive part is fortuitously connected to earth just by contact with the floor, or wall, etc. but this is a limited contact and may only allow a very low current to flow through the body because of the high resistance. However if the person were also touching a extraneous conductive part that was securely connected to earth (such as a metal water pipe leading underground or the like) then the resistance would be significantly lower and the person could receive a fatal shock.

This is where main bonding comes in. All the extraneous conductive parts, metal water pipes, metal gas pipes, metal structural steel, etc. that may be connected to earth are also connected to the main earth terminal with main protective bonding.

When a fault occurs there is a low resistance between the installation cpcs and the extraneous conductive parts, the only voltage drop will be from the resistance of the cpc back to the MET. This resistance is designed to be so low that the difference in potential between any conductive parts is less than 50V (The touch voltage), below this level it is considered unlikely that someone will receive a fatal shock.

Right at last to BATHROOMS

In a room containing bath or shower, or in similar areas like swimming pools and fountains, etc. then there is a good chance of someone being totally wet and this will reduce the resistance of the body to electric shock by virtue of improved contact with a potentially live part.
Therefore these areas are considered to present an increased risk of shock.
In order to reduce this risk the additional protection is by use of residual current devices that will disconnect before the current rises to a fatal level. However these also require the automatic disconnection of supply (by means of cpcs and protective devices) and reduction in touch voltage (by means of main bonding) to be present.

However it is also possible to reduce the risk by installing supplementary bonding to further reduce the touch voltage. To do this all exposed and extraneous conductive parts are electrically connected together within the location (or very near to it) by the supplementary bonding.
In this case the connection back to the MET is not relevant (but must be present for the cpc). In the case of a fault, instead of the length of the cpc back to MET being the resistance that determines the touch voltage, it is only the resistance of the cpc to the connection with the supplementary bonding that determines the touch voltage, since this is a lower resistance than before the touch voltage will be lower and therefore less dangerous.

To consider your queries:
For all 230V Class I equipment and fixed wiring there must be a cpc present if there is not then this is a danger and should be corrected. (if the fitting itself is class II then the cpc should not be connected to the fitting but terminated at the fitting)

“Low voltage downlights” these are extra low voltage (normally 12V) supplied by a SELV transformer. Because the supply is separated from earth no cpc should be connected to the fitting. Bu the cpc must be run to the point just before the electrical separation occurs (in practice there is a cable from the junction box to the transformer that does not actually have a cpc).

Where you have a light fitting with only line and neutral supply cables this may be a Class II fitting, if it has the square in square symbol, and should not have any metalwork earthed but the cpc should be taken up to the fitting and terminated there. (but it also may be from another country that does not meet the standards required for this country and therefore should not be fitted, but someone may have supplied the required cpc to ensure it is safe)

 

[Outspoken]

Richard, whilst what you are saying is correct, and very well written, I am not sure it would help GMjohn actually understand Electrical Potential and thus his understanding of Earthing. Although i am sure he will find your information very useful and informative and certainly something all practices sparks need to know and properly understand.

GMjohn, in order to assist you I have taken the time to knock up this little explanation for you of electrical potential. I hope you find it useful.

Voltage is electric potential energy per unit charge, measured in joules per coulomb ( = volts). It is often referred to as "electric potential", which then must be distinguished from electric potential energy by noting that the "potential" is a "per-unit-charge" quantity. Like mechanical potential energy, the zero of potential can be chosen at any point, so the difference in voltage is the quantity which is physically meaningful. The difference in voltage measured when moving from point A to point B is equal to the work which would have to be done, per unit charge, against the electric field to move the charge from A to B.

Fig 1: Used to calculate current in Ohm's law.

Fig 2: Used to express conservation of energy around a circuit in the voltage law.

Fig 3: Is generated by moving a wire in a magnetic field.


Fig 4: Used to calculate the potential from a distribution of charges.​

The Water Analogy:

A battery is analogous to a pump in a water circuit. A pump takes in water at low pressure and does work on it, ejecting it at high pressure. A battery takes in charge at low voltage, does work on it and ejects it at high voltage.

Fig 5:

​

In this type of analogy you can think of a closed Tap that has water behind it has infinite resistance, just as a socket with nothing plugged in also has infinite resistance because both prevent the flow of energy, mechanical and electrical respectively.

Earth Potential

In order to measure potential difference or Voltage with any real meaning we have to have a reference point that is common to all systems, and as the General Mass of Earth has an electrical potential in relation to true zero electron flow, and it is available to all (although not necessarily equally) it has long been a tradition to use the GME (earth) as this reference point from which all other potentials/voltages are measured.

By selecting the GME as this reference point is gives a common point that is accessible and easy to attain because one aspect of electrical systems is that to ensure they are at the same electrical potential you simply need to connect them together, in a simplistic way.

Please look at the following graphical examples of explaining the dangers and operation of electrical potential as a shock hazard to people.

Fig 6:
​

A person touching two different electrical services that are at the same electrical potential and, if AC, the same frequency and phase angle, will not receive an electrical shock

Fig 7:
​

However a person who is in contact with two sources of electrical energy that are at different frequencies or more importantly, two separate phase angles, will receive an electrical shock proportional to the electrical potential (Voltage) between the two sources of energy.

Fig 8:
​

Hopefully the above will allow you to be better acquainted with shock hazards caused by differences in Electrical potential and why the General Mass of Earth (Earth) is used for this purpose. In an AC supply system the Neutral conductor is, or should, be at the same electrical potential as the General Mass of Earth, it is also the point, by definition, at the source of the AC energy where the three phases of the supply are all at the same electrical potential (0V by definition but not reality).

Incidentally to all: Figures 1-4 are not my creation, I have them saved from another source (cannot remember where, had them years) but the all figures after that have been created using Visio.

 

[GMjohn]

Thanks for the excellent responses. I'm going to spend a few days digesting the posts and doing some more reading.