maximum ze for tt

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Mr Sworld

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Well the resistance of the earth rod should be 'as low as practicable'. This means below 200 ohms.

The Zs required to trip a 30mA rcd withing 0.4 seconds must be below 1533 ohms.

 

Mr Sworld

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Correct me if I'm wrong but thought it was Ra not Ze on TT?
Ra is the sum of the electrode and CPC resistances. Table 41.5 gives you the maximum Zs to satisfy the Ra x In < 50V equation. :)

 
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I've always worked on the principle that a good circuit within the property should comply with the mormal Zs values, adding Ze for the rod to get a total Zs value. However reading the regs suggests anything upto 1553 ohms is sufficient, which never felt right as this a huge heat generating resistance (worse than required to melt showers switches) ergo that last comment always struck me as nonsense, hence I work the way I do.

 

dean mac

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I've always worked on the principle that a good circuit within the property should comply with the mormal Zs values, adding Ze for the rod to get a total Zs value. However reading the regs suggests anything upto 1553 ohms is sufficient, which never felt right as this a huge heat generating resistance (worse than required to melt showers switches) ergo that last comment always struck me as nonsense, hence I work the way I do.
thanks guys

 

paulh

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hey Blinky

I don't know if I missed your point in which case sorry but someone will put me right ......A huge resistance means little current and little heat (hence touch voltages rising higher than 50v)thus no melted bits just a higher than could be sustained by the human body potential which will cause your heart to flutter.

As I said earlier sorry if i misunderstood your answer

 
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Funny you should mention that, cos I did some quick calculations after posting reply and have been waiting to be corrected since. Don't actually see how anything could work with high resistance and no ampage flowing, as for the heat, I was thinking of poor connections and arcing across????? Might be barking up the wrong tree, but kind of consisder that national grid is still trying to push electric into system at full potential, bit like a small crushed/broken pipe connected to reservoir of water - not much may trickle through to where its wanted, but force of water is still there trying to push through, so something has to give eventully.

Anyway got a turkey to stuff, Merry Crimbo to one and all.

 
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There can't be any resistance when there's no current flowing.

High resistance leads to the generation of heat. This is because the electrical energy is being 'held back' (resisted) by the material it is passing through. Simple physics and the conservation of energy means the energy can't be lost - it is converted into another form, in this case heat.

Whenever electricity is passed through a resistor (and to a greater or lesser degree anything is a resistor) heat is generated, often that is very small.

 

paulh

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Sorry to correct you apache but high resistance does not cause heat. heat is caused by the piece of equipment/joint/circuit not being able to handle the high currents associated with LOW resistances. As you stated high resistance eventually causes no current flow thus no trip of rcd's .

 
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Sorry to correct you apache but high resistance does not cause heat. heat is caused by the piece of equipment/joint/circuit not being able to handle the high currents associated with LOW resistances. As you stated high resistance eventually causes no current flow thus no trip of rcd's .
Just to be 100% accurate - a high current generates heat when passed through a resistor, anything the current passes through is a resistor - even the cable.

The science - when a greater current is flowing there are more electrons been exchanged (or excited) in the same area. The more electrons moving the more electrons collide and it is these collisions that change the electrical energy into thermal energy.

High resistance doen't cause heat in itself - only when current passing through! I know what I mean, but can see how I phrased it badly.

 
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Sorry Patch, but you have it wrong..

The value of the resistance determines the amount of current drawn.

The higher the resistance, the lower the current and vice-versa

eg 1V with 1ohm

I=V/R 1(A)=1(V)/1(ohm)

1V with 0.5ohms

2(A)=1(V)/0.5(ohms)

HEAT IS DEVELOPED BY HIGH CURRENT

 
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Sorry Patch, but you have it wrong..The value of the resistance determines the amount of current drawn.

The higher the resistance, the lower the current and vice-versa

eg 1V with 1ohm

I=V/R 1(A)=1(V)/1(ohm)

1V with 0.5ohms

2(A)=1(V)/0.5(ohms)

HEAT IS DEVELOPED BY HIGH CURRENT
I know - that's what I said

Just to be 100% accurate - a high current generates heat when passed through a resistor, anything the current passes through is a resistor - even the cable.The science - when a greater current is flowing there are more electrons been exchanged (or excited) in the same area. The more electrons moving the more electrons collide and it is these collisions that change the electrical energy into thermal energy.

High resistance doen't cause heat in itself - only when current passing through! I know what I mean, but can see how I phrased it badly.
 

paulh

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So......to recap apache. LOW resistances = high currents which means heat is generated.

High resistances = low currents and little energy dissapated which causes little heat.

Can we now move on!!!!!!!!!!! :) :)Guiness DrinkGuiness DrinkGuiness Drink

oh just remembered ....As mr sword says "Well the resistance of the earth rod should be 'as low as practicable'. This means below 200 ohms.

The Zs required to trip a 30mA rcd withing 0.4 seconds must be below 1533 ohms. "

]:) ] :) ]:)

 

davetheglitz

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Regarding heat generation there is essentially only one formula that counts:-

Power dissipated in a resistance = Current flowing through resistor I x voltage across resistor V

1) P = I x V

As I = V/R 2) P = V^2/R

As V = I x R 3) P - I^2 x R

In the above - if voltage is a constant then from eqn 2) if R decreases then power dissipated increases.

However - in most cases we are not talking about a single resistance in the circuit. Virtually every time we do a voltage measurement we are doing it in a potential divider - so for example we do a voltage measurement at an appliance taking a substantial current, then voltage will be dropped across the supply wires - so now voltage across the appliance is reduced and is no longer a constant. Plug this into equation 2, and the power dissipated will be less than expected due to the lower voltage.

Point being to say high current = high power dissipation is not necessarily true - it depends where you are measuring the dissipation from!

Either way - not much to do with TT systems.

My take is as follows:- I like RCD's in so many ways - but if you get a N-E fault then your system is severely compromised and there is no way for a customer to tell (unless they take sufficient current that 30mA flows down the earth conductor in normal use). My arguement here is that R1 + R2 should be sufficient that the fuse should blow quickly enough if there is a N-E fault and an earth fault. However the R2 value could be surprisingly large if the N-E fault is on the lighting circuit - so no idea what a good compromise is here!

Don't know if this has helped - but hope all had a good Christmas.

 

laserdavy

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Actually I was told it was 1666 ohms :(

ie 50v/30ma

But it should be less than 200 ohms to account for ???? the weather in Guatamala :p :p

 
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Actually I was told it was 1666 ohms :( ie 50v/30ma

But it should be less than 200 ohms to account for ???? the weather in Guatamala :p :p
Table 2.3, page 48 Guidance note 3, Insp & Testing.

30mA RCD Ra max 1667

100mA RCD Ra max 500

300mA RCD Ra max 167

500mA RCD Ra max 100

200ohms Ra is a practical guide value for when doing Ra in good soil conditions...

it would allow for deterioration in soil drying/freezing etc..

(paragraph below table 2.3;))

I Remember a fair while back doing a test on a Rod in December...

ground was frozen.. around -3 or less...

Ra was reading 700ohm+!! earlier reading had been some where around the 180 - 190 ohm..

In fact it would be interesting to know of any big variations anyone has experience on the same rod over different seasons of the year???....

:)

I may even be tempted just to go a stick a rod in me own back garden as a reference test rod to see how it changes??? ?:| ? :|

 

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