Is it ever acceptable to exceed voltage drop limits if Zs is still within limits of OCPD?

[Spark123]

So you have heard of the 80% correction for temperature? 1/1.24= approx 80%

As for the ohms law resistive things mentioned before, we have to remember a lot of resistive loads create heat with a positive temperature coefficient (which balances out) so it isn't a linear response for them with respect to resistance at different voltages.

 

[EFLImpudence]

So you have heard of the 80% correction for temperature? 1/1.24= approx 80%

Yes, but I have only ever come across 80% and 1.2.

As 1.2 is correct for my circumstances (20°), I assumed the OSG (as is their wont) were just approximating to 80%.
It would not be that critical normally but as eveares was asking about limits...

 

[ericmark]

It just happens my son works were new applicants are set a test before being taken on. He sent me a picture of the test paper with the reply to a question on volt drop I assume for my amusement, not name on part of paper copied.

Explain the cause of voltage drop within an electrical installation.

It’s mainly due to gravity, where a cable runs downwards. The voltage tends to drop and collect towards the bottom. With modern pre-insulated cables it is more of a problem and easy test. Measure voltage in your upstairs sockets and compare it to those downstairs – The downstairs voltage will be higher. Volt drop is not generally a problem in Bungalows.

It would seem from that reply we have all been getting it wrong? Georg Simon Ohm must be turning in his grave.

 

[EFLImpudence]

Explain the cause of voltage drop within an electrical installation.

It’s mainly due to gravity, where a cable runs downwards. The voltage tends to drop and collect towards the bottom.

Install it running upwards, then.

With modern pre-insulated cables it is more of a problem and easy test.

Twas a bugger insulating your own cables before that.

Measure voltage in your upstairs sockets and compare it to those downstairs –

You must do it at the same time before more drops down.

The downstairs voltage will be higher.

How can anything downstairs be higher than upstairs?

Volt drop is not generally a problem in Bungalows.

That's because bungalows don't have a downstairs.

 

[ericmark]

These people who think they know it all, are especial annoying to those of us that do.

 

[Simon35]

The MCB protects against faults in the cable. So the type of load is not part of the equation.



I do wonder if specifying a maximum acceptable voltage drop has anything at all to do with the calculations for sizing cables and MCBs in over current fault protection. With a dead shirt fault the voltage at the fault is zero. 100% drop and not the 10% or 3% used in cable size calculations.

A dead short to the CPC in the cable at some point along the cable? In that case the voltage at the fault wouldnt be zero.

 

[eveares]

So why did you prattle on about SMPSUs which are a complete irrelevance?

Please provide a well argued and intelligent explanation of how it was either useful or helpful.

To end this stupid debate once and for all;

SMPS's are irrelevant for obtaining Zs readings as while the fault loop impedance and thus Zs of the active circuit is going to be dependent on the input voltage and current draw (to maintain the rated output) and thus SMPS's are not fixed impedance devices (Ohms Law R = E ÷ I ); Zs is about ensuring the loop impedance is low enough for enough current to "flow" in order to operate the OCPD and is not about the current loop impedance of a active circuit under fault.

And with a fixed impedance, if the voltage drops, the current, now think, yes, that drops too.

That's just Ohms Law. Again R = E ÷ I.

 

[JohnW2]

I'm late in getting into this thread, but ....

Sure the current will drop as the voltage drops for pure resistive loads due to Ohms Law, and thus the current and time required to reach the required limit to operate the OCPD will go up.

Not both - the current required to operate the OPD within any given time period is a constant, dependent only upon the device concened (e.g. 5 x In for a Type B MCB). Hence, if current falls, then the time taken before the OPD operates will increase.

So if you get Zs from Ze + (R1 + R2) and it is within the limits of the OCPD for the 0.4s disconnection time, would it still not operate within the 0.4s disconnection time providing voltage drop for the design load did not exceed 25.3V ... (25.3V is difference between 230V and 230V-11% [6% Nominal + 5% permitted Voltage Drop ])

Not necessarily. If, say, the Zs was only just within the required limit to result in OPD operation within the required time when the voltage was 'nominal', then any reduction in voltage (hence current, under fault conditions) would result in the OPD no longer operating as rapidly as required.

Kind Regards, John

 

[ericmark]

With a type B we are told 3 to 5 times rated value, and we are also told to allow for volt drop, not got the amendment 3 so can't remember exactly how much to allow for volt drop. But assuming a 0.35Ω incomer and working with a B32 we have a value of 0.94Ω centre of a ring to satisfy the volt drop. That is a long way from the 1.368 value which I think replaced the 1.44 value. So without being just on the limit one could allow quite a bit over the 5% volt drop and still be within the trip limit for a B type and very likely the B type will not be bang on 5 times more likely somewhere between 3 and 5 so again even at 1.44Ω as it was still likely it will trip.

The problem is a trip is never tested. We test RCD's but not MCB's so a faulty one can remain for years and years and no body knows it is faulty. I remember at a caravan site 1990's the main fuse was blowing, but all caravans had just a 5A the old Loadmaster MCB which should not allow an overload. So we fitted a 16A plug to a 2.5 kW fan heater and one by one tested the MCB's to see if they tripped. We ended up with a box of faulty MCB's on closer inspection we found the adjusting screw tampered with and a few 16 amp ones with the 1 scratched off it seems there had been an attempt to get more power by many of the workers stopping on the site. That is however the only time I have actually tested MCB's and even then not the magnetic part of the trip.

 

[JohnW2]

With a type B we are told 3 to 5 times rated value, and we are also told to allow for volt drop ....

I'm not sure where you got that from. The requirement is for the ('zero impedance fault' at furthest point in circuit) fault current to be at least as high as the trip threshold of the OPD. That used to be calculated (from Zs, which includes Ze) assuming nominal supply voltage but, since the appearance of Amd3, it is now calculated assuming 0.95% of nominal supply voltage (not quite low enough to cater for the lowest permissible supply voltage, which is 0.94% of nominal!). That calculation implicitly takes into account what voltage drop will occur under fault conditions, but there is no requirement to make any additional 'allowance' for voltage drop (which would not really make any sense).

Although a Type B MCB may trip at as low as 3 x In, the calculations obviously have to be done for the 'worst case' (of an in-spec MCB) of 5 x In. As you say, and as often discussed, there is a problem in that we have no practical way of testing an MCB (and certainly not a fuse ) to see if it is performing according to spec.

Kind Regards, John