Everyone seems to have missed that Bernard has this the wrong way round.
Edit - except Stoday.
I had assumed that was why nominal voltage is low.
Namely, if satisfactory at 230V then even safer at 'usual' voltage.
Part P / BC question
- Thread starter coloumb
- Start date
Edit - except Stoday.
I had assumed that was why nominal voltage is low.
Namely, if satisfactory at 230V then even safer at 'usual' voltage.
Sponsored Links
H
holmslaw
..
You've certainly lost me there! If it's a 'simple load' (i.e. one which simply obeys Ohm's Law), increasing the voltage will increase current - and hence become 'less safe' (in terms of cables etc.).
Are you perhaps thinking of (unusual, domestically) 'fixed power' scenarios, in which a higher supply voltage would result in a compensatory reduction in current so as to achieve the same power?
Kind Regards, John
.
Apologies
John - No, obviously wasn't thinking at all.
Apologies
John - No, obviously wasn't thinking at all.
Sponsored Links
H
holmslaw
..
P
Paul_C
Maybe you should market the idea as one of those "Go green" boxes - Plug this device into a socket and it will raise your household voltage to 260V in order to save energy!
How about one which raised the voltage to 1000V - that would probably reduce one's electricity bills to zeroMaybe you should market the idea as one of those "Go green" boxes - Plug this device into a socket and it will raise your household voltage to 260V in order to save energy!
Kind Regards, John
P
Paul_C
How about one which raised the voltage to 1000V - that would probably reduce one's electricity bills to zero
I'm sure it would! Not so sure about the bill for replacement appliances though......
If an appliance is to be connected to a real circuit, it's implicit that it should operate safely if the voltage should rise to 253V or fall to 205V. If it overloads when it's supplied at over 230V, it's faulty.
A base of the nominal supply voltage is sensible. Mrs Jones wants to buy a 100W lamp, not a lamp that might be 121W or 79W or something in between.
It's impossible for an electrician to know any value for the voltage of supply for a new connection other than the nominal voltage. He can only guess at 240V or some other voltage. So it's only sensible to use the nominal voltage.
I like the notion that everything should be based on the maximum voltage. Then the power of a 10kW shower would normally be 9.2kW and could fall to 6.6kW at times of peak demand. Would consumers complain? You bet they would. Would they get anywhere? Have slow broadband consumers got anywhere?
A base of the nominal supply voltage is sensible. Mrs Jones wants to buy a 100W lamp, not a lamp that might be 121W or 79W or something in between.
It's impossible for an electrician to know any value for the voltage of supply for a new connection other than the nominal voltage. He can only guess at 240V or some other voltage. So it's only sensible to use the nominal voltage.
I like the notion that everything should be based on the maximum voltage. Then the power of a 10kW shower would normally be 9.2kW and could fall to 6.6kW at times of peak demand. Would consumers complain? You bet they would. Would they get anywhere? Have slow broadband consumers got anywhere?
I don't think anyone is going to disagree with that. The discussion is not about the ability of the appliance to operate satisfactorily and safely over the full possible range of supply voltages - but, rather, about what conditions (i.e. current consumption) should be used for designing the circuit to supply it. I would suggest that normal 'safety' practice would be to design on the basis of the worst-case scenario - in this instance, maximum permissible supply voltage.
See above - no-one is disagreeing with that. Mr Jones is, as you say, likley to want a 'single figure'. Whether it makes sense for that figure to be one which is probably 'low' in relation to what most consumers experience for most of the time is another matter - but not what we are discussing.
As above, when undertaking safety-related aspects of design, there would surely be a good argument for designing a circuit for 'worst case' conditions? If the designer is theoretically 'gambling' by designing for something (arbitrary) less than the 'worst case', one again might question the wisdom of designing on the basis of a voltage which is probably lower than that experienced by the most installations for most of the time.
Again, we're not really talking about the 'rated power' (or whatever) of appliances - but, rather, the safe design of circuits to supply them. From the users' viewpoint, it would probably be most sensible for the 'rated power' to relate to some sort of average of voltage supplied to UK installations - but people will probably get used to whatever convention is adopted, so long as it is consistent.I like the notion that everything should be based on the maximum voltage. Then the power of a 10kW shower would normally be 9.2kW and could fall to 6.6kW at times of peak demand. Would consumers complain? You bet they would. Would they get anywhere? Have slow broadband consumers got anywhere?
Kind Regards, John.
Exactly so!
The value convention has chosen is the nominal voltage. Has worked well, apart from one exception. (gives the impression that CFL equivalent watts are dim).
I would suggest that the normal design practice is to optimise a design to meet typical conditions and then to add contingencies to cover exceptional conditions. This applies in general, civil as well as electrical engineering, electronics as well as power.
Designs based on a worst case are generally bad because of inconsistencies. For example, what is the worst case allowance for diversity? If one designer proposes four ring finals in a house and another two in the same house design, does that change the worst case current? When the appliances to be connected to the installation are unknown, what is the worst case that's consistent between designers?
If you design to meet the worst case, shouldn't you test to the worst case too? The classic is Chernobyl, where the worst case situation was simulated by shutting off safety mechanisms to check that the reactor was still safe. Oops!
The protective devices should operate and be tested at a level that is lower than the predictable and calculated worse case level.
The cables and fittings however should be capable of withstanding the worse case situation or fail in a safe way.
That was complicated by the operators not reporting the dangerous situation for fear of reprisals for daring to suggest there were faults in the design of the control equipment and the design of the control room procedures. In that type of regime one does not offer even constructive critique of the actions of one's superiors if one wants to remain employed.
You are saying exactly the same as me, but using slightly different phraseology. Your 'adding contingencies to cover exceptional conditions' is exactly the same as my 'design on the basis of the worst case scenario'. I said nothing about 'optimising design', which is a totally different matter, and I agree that should be done for the 'typical conditions' - although I could obviously question whether 230V is the correct 'typical condition' to assume in the UK at present (and for the forseeable future).
Kind Regards, John.
Edit: Typo corrected
Diversity is a statistical concept which is not only a designer's nightmare (beacuse it stops them doing 'proper design') but would not be allowed in seriously safety-critical situations. However, it is a very useful concept, provided that there are suitable protections in place (see below).
That's why it is a designer's nightmare. However, as above, the existance of an OPD in the circuit prevents danger when (as will sometimes happen - such is a statistical approach) loads 'happen' to exceed the maximum estimated on the basis of applying diversity. The dangerous situation would be to apply diversity but not have an OPD to over-ride it with a 'ceiling'
In a safety-critical environment, one would tests to the extent that is practical without being destructive. I would suggest that the greatest deficiency of standard testing of electrical installations is that there is no practical way of testing the correct operation of OPDs - since it is they they usually represent the bottom line of ensuring safety in 'extreme conditions' (of load).
Kind Regards, John.
In response to JohnW, why is diversity a designer's nightmare and what's the proper design you refer to?
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
Sponsored Links
