It does not mean that I am recommending what is shown on a picture as a good or correct way to do something, unless I specifically say so, of course.
Beginners, please take note. The pictures that I show on this blog are materials for you to see and think. These are real life pictures from real construction sites, and construction industry is a really complex world by itself.
Picture 1 – The earthing of temporary supply DB using the cable’s wire armor
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Picture 2 – A closer view on the termination of the cable armor
The safe utilization of a temporary electric supply often depends on the existence of an effective electrical earth.
It is the responsibility of the person in charge of a construction site to ensure that the earthing of the electric supply system is working effectively. The site’s electricity supplier (the electric supply authority) has nothing to do with this part of the electric supply.
Many electricity supply authorities use protective multiple earthing (PME) system. With this method, the electrical system’s neutral and earth are combined.
When this method is used, all metalworks including structural metalwork must be bonded together in such a way to make all metal parts electrically continuous.
However, in most real life construction sites, this bonding is hard to do and almost impossible to maintain properly throughout the duration of the construction period.
So usually, the public utility supplier will not connect the supply until an alternative electrical earth is provided and fully tested.
They also insist that evidence of the earthing test results endorsed by a competent electrician is submitted together with the completed application forms for the temporary electricity supply to the site.
Independent earth electrodes
Several methods are used to provide the alternative electrical earth. The most commonly used method is to use independent earth electrodes installed at the location near the main intake temporary switchboard (this is usually the location where the temporary meter is installed).
This will ensure that the protection fuses will operate and disconnect the site electrical installation from the incoming public supply in the event of a fault.
This disconnection is an absolute must in order to minimize the damages to the site’s electrical system due to the fault.
It is also to prevent the fault from spreading upstream to the electric supplier’s distribution network. The latter is the main reason the electricity supplier is worried about this aspect of the temporary installations.
Picture 3 – Temporary electric supply: A poor man's complete system
I will devote a whole post in the future for this subject of earthing.
For now, I have a few picture of temporary electrical grounding for your viewing pleasures. I seek an apology from those readers who already have some electrical knowledge. These pictures are not for you guys. They are just too simple.
However for those managers and construction people who have always been intimidated by those colleagues on these sorts of issues, let me assure you that if you understand what these three lousy pictures say, then you already understand what an electrical grounding is.
Picture 4 – Temporary incoming cables, kWh meter, DB, earth wire and protective PVC conduit
Picture 5 – Temporary earth grounding
Now lets start with picture 10. You may need to click on the picture to see it in full size.
The black case on the wooden panel there is the temporary electric meter supplied by the electric authority. Below it in white PVC casing is the temporary distribution board.
You can also see a piece of wire in green insulation coming out downward from the distribution board.
This wire goes straight into a two-meter length white PVC conduit, and (if you watch it closely) it comes out of the vertical PVC conduit at the bottom and goes straight into the concrete floor just beside the plywood partition wall.
All these you can see much more clearly in Pictures 11 and 12.
Now let’s get back to the black energy meter.
Above it you can see two lengths of black insulated cables coming down from high level (below the roof level of the temporary wooden structure).
One cable terminates at a black piece of component just above the meter, while the other one terminates at a cheap white plastic terminal block.
The two components each have a cable of the same size coming out at the bottom connection and terminate at the temporary kilo-watt-hour meter.
Now lets get a little bit more technical.
Single-phase electrical installation
This is what we call a single-phase electrical installation (a three-phase supply would have four black cables coming down from the roof level – three phase cables and one neutral cable).
Therefore, the incoming supply from the authority is also a single-phase supply (we apply single-phase supply, so they give us a single phase supply).
An electric supply from the authority can be one of a few types.
Single-phase two-wire system
The type as recorded in the pictures is the single-phase two-wire type (three-phase 4-wire if we apply three-phase supply). This type will have two incoming cables from the authority’s distribution network – one live or phase cable and one neutral cable.
Other types may have three incoming cables from the distribution network for the same one-phase supply – the same live and neutral cables, plus a third cable, which is the earthing cable.
A construction site needs an independent, reliable electrical earthing
Now as I said earlier in this post, a construction site really needs an independent and reliable earthing, an electrical grounding that is not dependent on any third party’s grounding or even the electricity supplier’s grounding.
Failure to provide this can result in fatal injuries from electric shocks, even multiple casualties in a single accident.
Never mind my emotions on this (if you can sense them), but an electrical accident is that dangerous and electrical shocks can strike silently without warning.
Because of the need for that independent and reliable earth for the temporary site supply, the green electric cable is installed below the white PVC distribution board (Picture 11 shows this more clearly).
What if this earthing is not working?
The electrical earthing is important so that the electric shock protection device (ELCB or RCD) can operate and isolate the incoming supply from the temporary installation. (I have uploaded a few ELCB pictures at this post, 1-Phase ELCB Connection Pictures, for the readers who wish to see them).
Without this cable, the electrical system can still work. The workers can still use their tools and do a good job for their employers so the construction venture can end up being very profitable for the shareholders of the company.
However, one fine day, the extension cord that carries current to the electric drill one of the workers has been using gets damaged, exposing the live wire to an unintentional contact with any of the workers around the area.
Being hard at work, the workers bodies and clothes are usually very damp or very wet with sweat, and human sweat is a good conductor of electricity just like the electrolyte in your car battery.
In this situation, all that is needed is one accidental contact at any part of a worker’s body with the exposed live conductor of the damaged extension cord. Then he would receive a severe electric shock injury. Even death is highly likely depending on where on the worker’s body the contact to the live wire happens.
Severity of electric shock injuries depend on the location of the contact
If the contact is at the hands, then the electric shock current will travel through the chest and the workers heart before going down to the legs and the ground. Then you may have a case of fatal injuries there.
Sorry for the diversion. This electrical earthing matter is so simple that I have to drag the stories into the injury aspects to make it a bit longer ;-)).
Not only it is simple, it is also plain cheap and low cost.
Look at the picture again. You have a short length of the green wire and two meters of the white PVC pipe.
Wait… I know what a few of you are thinking…
Yes, there is copper earth electrode in the concrete, which goes straight down below the concrete about 1.5 to three meters into the ground.
Cheap in price, costly in lives
The above example is a very small installation, so it looks simple. This wooden structure only draws a few amperes. However, even for a large installation, the grounding is relatively just as simple.
The point that I am trying to make in here is that providing an independent and effective earth is not a challenge at all in most construction site situations.
So, do not risk human lives just to save some construction cost there.
Update February 11, 2010:
The challenge of a site electric supply
It is always a challenge to provide the electricity supply to the site people safely. Construction sites are among the most challenging environments to the safe use of electricity.
A lot of works are done outdoors, in all sorts of weather conditions. Wet and damp conditions present very high risks of severe electric shocks.
The workflow of site works is constantly changing as the construction work progresses. Therefore, the temptations to improvise the electrical distribution system are often too great to resist.
Routine construction activities, the demolition and excavation works may all result in damages to both the temporary supply and the newly fixed permanent supply systems.
When the activities at a site are at its peak with hundreds or thousands of workers ,not to mention construction vehicles and machineries, the site usually become congested.
This sort of situations makes the control of risks very difficult. Temporary cables and electric construction tools and equipment are very likely to be damaged by the movement of heavy machines and materials.
The people who use electricity at site have various needs, and sometimes conflicting interests and expectations. The workers and team leaders themselves work for different subcontractors and suppliers.
In order to maintain any reasonable degree of safety and control of risks, a effective site management is an absolute must-have for the site main contractor or the client’s people who are in charge at the construction site.
Due to the nature of construction works, a risk-free environment is impossible to attain most of the time. However, some risks can be avoided by careful planning before the work commences at the site.
Protect the earthing system
The protection of workers against electric shocks and injuries depends so much on the integrity of the earthing part of the electrical grounding system.
The earthing part, as shown in the above pictures, includes the grounding electrodes and all the underground conductors or cables that form part of the earth grounding network.
However, that is easier said than done.
These networks of electrodes and the connecting conductors are almost always installed at the ground level. They can easily be damaged by uncontrolled excavation works construction vehicles.
The personnel in charge of safety aspects of the construction site should include it their daily routing to patrol this part of the site temporary supply installation.
Other than that, it always a good idea to install some highly visible warning signs to warn workers and drivers of construction vehicles of these critical electrical supply safety parts.
The following are two pictures show the temporary warning signs put up by the contractor indicating the location of the temporary earthing electrodes.
They rushed up to install this after I found out in my second day of assignment at the construction site that the temporary supply at the building under construction has been operating for many months without earthing connection at all.
I have sent a post earlier about this temporary installation. Read it here, Temporary Electrical Panel Pictures.
Picture 6 – Temporary electrical earthing warning sign
Picture 7 – Notice the temporary supply cables at the fence, but that is a topic for another post.
There were 250 to 400 construction workers at site daily. Luckily there was no electrical accident yet. Really, really lucky.
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