Understanding the relationship between TNCS, TNS and TT

[HappyHippyDad]

I have never actually had to make an electrical system into a TT. I have added rods in order to get an effective Ra but never had to (for example) install a TT on a new garage or shed.

It's best I give an example with regards my question..

A new garage has a supply taken to it from the house. The house is PME and this has been extended to the garage, lets say 4mm 3 core SWA. The 3rd core is perfectly adequate as an earth. Also, the installer puts a copper rod in the ground and attaches this to the earth bar in the new garage CU. Is this OK? Is this actually better? Is it now dangerous?

I've got a few ideas but I'd rather not influence the line of thought and just leave it as above.

Cheers all.

 

[Cookie]

The particular point that Edmond and I had picked up on was a specific statement that has cropped up a few times on the forum regarding USA installations. In cases where a homeowner has reported voltage fluctuations, especially where overvoltage between one hot and neutral of a split-phase installation is occurring and indicative of a high-resistance PEN, a recommendation has been made to check / repair the ground rod connection.

As per my post #24, that seems unlikely to do anything at all, and if it does, is surely concealing the actual fault.

Often power company techs ignorant of how electricity works will advice electricians and HOs to re-check their grounding when high/low voltage is measured at the service. Of course the rest of us know this would be the symptom of a high resistance or open service neutral. The battle is then getting the power company to come out and load bank test their service.

 

[LastManOnline]

Often power company techs ignorant of how electricity works will advice electricians and HOs to re-check their grounding when high/low voltage is measured at the service. Of course the rest of us know this would be the symptom of a high resistance or open service neutral. The battle is then getting the power company to come out and load bank test their service.

So would it be correct to say that your current arrangement (two 8 foot rods) does not really provide an effective alterative path during an open PEN situation? If not What in your view is the solution?

 

[Cookie]

So would it be correct to say that your current arrangement (two 8 foot rods) does not really provide an effective alterative path during an open PEN situation? If not What in your view is the solution?

Two rods typically do little for an open neutral, however water pipe grounding can and often does mask an open neutral. Its not uncommon for water line workers to carry a pair of jumper cables for this exact reason.

If I had my way all loads would be connected phase-phase with the PEN simply taking the function of a PE. RCDs can cover thereafter. However, the majority of the world does not share my views.

 

[davesparks]

If I had my way all loads would be connected phase-phase with the PEN simply taking the function of a PE. RCDs can cover thereafter. However, the majority of the world does not share my views.

That will only work if you have at least two phases in your supply. Plus if course it makes the PEN pointless and it woukd just be a PE.
Which then defeats the cost saving advantage of using TNCS as you will be back to using 3 core cable instead of 2 core.

 

[LastManOnline]

Two rods typically do little for an open neutral, however water pipe grounding can and often does mask an open neutral. Its not uncommon for water line workers to carry a pair of jumper cables for this exact reason.

If I had my way all loads would be connected phase-phase with the PEN simply taking the function of a PE. RCDs can cover thereafter. However, the majority of the world does not share my views.

"Two rods typically do little for an open neutral". Well that certainly puts the single 4 ft rod required in most TNC-S countries in perspective. I spoke with an ex inspector recently who described the rod's we use here as "fig leafs".
Your solution is basically a form of TNS which is without doubt the best electrical system.
While the "rest of the world may not share your view", it does, nt alter the fact yours is correct
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That will only work if you have at least two phases in your supply. Plus if course it makes the PEN pointless and it woukd just be a PE.
Which then defeats the cost saving advantage of using TNCS as you will be back to using 3 core cable instead of 2 core.

Whatever "cost saving advantage" there is using TNC-S was always short-term, me thinks. I can understand turning to it as a "stopgap" measure, but long term? was that ever really an option?

 

[pc1966]

Whatever "cost saving advantage" there is using TNC-S was always short-term, me thinks. I can understand turning to it as a "stopgap" measure, but long term? was that ever really an option?

Traditionally we have though of the supplier's Earth as trustworthy and never had to consider it going to 100V+ during a fairly basic fault when installations were being designed. It retrospect they would have been better cost-saving by pushing for TT in place of TN-S

 

[Vortigern]

Well then time for another device to sardine into our CU, an overvoltage device!

 

[Simon47]

Shhh, don't give them ideas !

 

[Cookie]

"Two rods typically do little for an open neutral". Well that certainly puts the single 4 ft rod required in most TNC-S countries in perspective. I spoke with an ex inspector recently who described the rod's we use here as "fig leafs".
Your solution is basically a form of TNS which is without doubt the best electrical system.
While the "rest of the world may not share your view", it does, nt alter the fact yours is correct

I had to smile at this! Not many are willing to fully accept the advantages put forth by TN-S without the neutral distributed.

Whatever "cost saving advantage" there is using TNC-S was always short-term, me thinks. I can understand turning to it as a "stopgap" measure, but long term? was that ever really an option?

Long terms if a disaster to the goals of the regs: protecting life and property. What many people fail to realize is that a large number of electrical practices stem from when countries where at war, recovering from war or struggling economically. Engineers, electricians and code experts were forced to improvise, often picking the lesser of several evils.

For example, during WWII the US made an exception that ranges and dryers could ground through the neutral to save on copper for the war efforts. The practice did not end until after 1996.

Today however we can look past a few extra cents for what is significantly safer and more economical in the long term.

 

[Cookie]

Well then time for another device to sardine into our CU, an overvoltage device!

Just one of the many hazards the regs do not address. Dealing with neutrals involves much, much more complexity to mitigate those risks- risks which often can not be fully mitigated.

 

[Lucien Nunes]

For example, during WWII the US made an exception that ranges and dryers could ground through the neutral to save on copper for the war efforts. The practice did not end until after 1996.

TN-C with PEN all the way to the load was popular elsewhere too and is still in widespread use in places like former Soviet bloc countries. One little screw holding the incoming PEN to the metal casing of the panel, and various load neutrals run back to it. I recall being in Germany in the late '80s and struggling to overcome the mental block on connecting the metal body of some fluorescent lights to the grey (neutral) wire on an ungrounded lighting circuit.

For me the weak link in dryer / stove thing was the 10-30 and 10-50 receptacles. The PEN is much more likely to be disconnected or high-resistance when it goes through a single prong of a plug and receptacle, than when it's a continuous wire up to the receptacle but then splits to carry N & E through two separate prongs as far as the dryer. In any case I think it's a different category of hazard than a DNO's PEN.

 

[Cookie]

TN-C with PEN all the way to the load was popular elsewhere too and is still in widespread use in places like former Soviet bloc countries. One little screw holding the incoming PEN to the metal casing of the panel, and various load neutrals run back to it. I recall being in Germany in the late '80s and struggling to overcome the mental block on connecting the metal body of some fluorescent lights to the grey (neutral) wire on an ungrounded lighting circuit.

For me the weak link in dryer / stove thing was the 10-30 and 10-50 receptacles. The PEN is much more likely to be disconnected or high-resistance when it goes through a single prong of a plug and receptacle, than when it's a continuous wire up to the receptacle but then splits to carry N & E through two separate prongs as far as the dryer.

The hazards are the same. Remember that connections/splices can and do fail- especially when they are carrying current.

In any case I think it's a different category of hazard than a DNO's PEN.

While the risk is reduced, a major hazard still exists.

 

[Simon47]

Interestingly, I was looking for something else and came across ENWL document - Code of Practice 332, LV Service Connections and Application of PME. It goes into great detail as to when different configurations may or must be used, who pays when something must be changed, etc.
There really are some interesting, and in places, eye opening, facts to be gleaned from it. Far too much to try and summarise it, but if you're interested in the practical aspects of PME from the DNO's PoV then it's a very interesting read. There's a lot of network configuration mentioned that I don't think have been considered in this thread.

Clearly PME isn't quite as haphazard as some people seem to think it is.

 

[LastManOnline]

Interestingly, I was looking for something else and came across ENWL document - Code of Practice 332, LV Service Connections and Application of PME. It goes into great detail as to when different configurations may or must be used, who pays when something must be changed, etc.
There really are some interesting, and in places, eye opening, facts to be gleaned from it. Far too much to try and summarise it, but if you're interested in the practical aspects of PME from the DNO's PoV then it's a very interesting read. There's a lot of network configuration mentioned that I don't think have been considered in this thread.

Clearly PME isn't quite as haphazard as some people seem to think it is.

Appreciate that and look forward to looking into it.

 

[Cookie]

Interestingly, I was looking for something else and came across ENWL document - Code of Practice 332, LV Service Connections and Application of PME. It goes into great detail as to when different configurations may or must be used, who pays when something must be changed, etc.
There really are some interesting, and in places, eye opening, facts to be gleaned from it. Far too much to try and summarise it, but if you're interested in the practical aspects of PME from the DNO's PoV then it's a very interesting read. There's a lot of network configuration mentioned that I don't think have been considered in this thread.

Clearly PME isn't quite as haphazard as some people seem to think it is.

Does TN-S actually exist anywhere in practice?