Adding an earth rod IN ADDITION to a TN system

[HappyHippyDad]

I have just watched one of John Ward's interesting videos -

It all makes sense apart from one of the final points involving adding an earth rod to an existing TN system. Not making a TT, but using the rod in addition to the TN system. This part is at 20 minutes and 8 seconds if you want to skip to it.

In relation to a broken PEN conductor he mentions that a suitably low Ra would be required for this rod to be of use. He mentions 10Ω's or less. He then says it may need to be 4Ω's or less if you have a substantial load on the system (i.e EVCP). I realise this is so a dangerous voltage is not reached but I cannot see how that relates to his figures of 10Ω and 4Ω.

For example. Lets say we have 20A (not even a substantial load) flowing throw the system (with a PEN fault) and we have a rod with an Ra of 10Ω

Using V = IR

V = 20 x 10 = 200V

I realise I must be using ohms law in the wrong context here as if the load was 40A the equation gives 400V which is impossible on a single phase system. Confusing!

 

[pc1966]

As folks have pointed out many time, a single rod will make little difference but if you have a dozen or two homes on a faulted segment then all those rods is getting down to a decent Ra. Also they are probably achieving some vague semblance of balance on the 3-phase you might only have low tens of amps imbalance to deal with and not the total of hundred-ish or so that simply adding the typical house values up gives.

The USA has had this for a while but not a very good specification (two rods of given size, no spec on Ra) and the ROI has been doing this with TN-C-S

It is a shame that AM2 did not go with the requirement for new properties to have foundation rebar bonded, etc, as that would get decently low Ra in most cases.

 

[timhoward]

Also they are probably achieving some vague semblance of balance on the 3-phase you might only have low tens of amps imbalance to deal with and not the total of hundred-ish or so that simply adding the typical house values up gives.

It's only just occurred to me that while this is a clear benefit, there is also technology relying on phase imbalance and arising volt drop/raise to detect the condition in the first place, e.g. the indent 4 EV chargers and Matt-e devices who look for <207 or >253V. In theory could the phase balance improve to a degree that these devices wouldn't function?

I actually feel it should be the DNO's responsibility to deliver an acceptable supply with an acceptable number of real earth connections rather than pushing the onus onto the customers to add electrodes to support the network.
If it get's 'fixed' at the point of the customer installation then that precludes any future technological advances for detecting the conditions, e.g. micro-controller based monitoring typical voltage ranges, maybe even smart connected and monitored, or going back to a form of the VOELCB technology to detect a PEN-real Earth potential difference.

I accept that once a critical mass is reached this will make touch-voltages a lot safer, but my fear is that it will hide the problem better as well.

 

[loz2754]

It's only just occurred to me that while this is a clear benefit, there is also technology relying on phase imbalance and arising volt drop/raise to detect the condition in the first place, e.g. the indent 4 EV chargers and Matt-e devices who look for <207 or >253V. In theory could the phase balance improve to a degree that these devices wouldn't function?

Indeed
This is precisely why version 2 of the Zappi charge point also has a separate CT to monitor the earth cable as well as built in OPen protection.

 

[davesparks]

It's only just occurred to me that while this is a clear benefit, there is also technology relying on phase imbalance and arising volt drop/raise to detect the condition in the first place, e.g. the indent 4 EV chargers and Matt-e devices who look for <207 or >253V. In theory could the phase balance improve to a degree that these devices wouldn't function?

The devices will function as their purpose is to prevent danger arising from an open PEN fault. If other protective measures have already reduced the danger to a level where these devices don't detect it or operate then they aren't needed to operate at that point.

 

[Deleted account]

I’ve just mentioned those John ward videos on another thread. he is brilliant.

the only problem I have with this particular video is he explains how to do it then explains how it may not work. leaving me more confused and out of my depth that I was before.

However his explanations are brilliant and easy to follow and understand, but designing new circuits and deciding on the earthing systems is becoming a nightmare.