Safety of Using Adiabatic Method?

[Cookie]

I'm thinking, and while I could be wrong, is the adiabatic method a possible latent safety hazard? As well as undersized CPCs on circuits 32 amps and under?

Normally DNO sources are strong with relatively high PFC, and we have a known Ze value. However, should someone bring a portable generation unit on site after an outage, or a large yet short circuit limited inverter (Tesla Power Ball), Ze values can be such that MCB opening time increases. Such an increase can overheat the CPC, up to the point of melting it.

From a touch voltage perspective I humbly do not see a major concern however, in that 20 x 10 = 200 amps, 1.15 ohms or 46kw- a 46kw load on a 25kw generator would result in a significant drop in the output voltage whereby 230 volts dropping to 110 volts giving 55 volts to remote earth such that a longer disconnection time will not result in organic bodily injury.

However, the risk of fire and shock from an overheating CPC is relevant.

What do others make of this point of view?

 

[pc1966]

Many generators have RCD on the output for this sort of reason, as well as protection for socket outlets against shock. Typically don't have the PFC in reality to trip things (even if they measure as "stiff" due to electronic regulation on they usual style of impedance meter).

(We are looking at a generator in due course, probably 10-20kVA 3-phase and probably will ask the supplier for a delay 100mA RCD and hard-wired terminals instead of socket and 30mA "instant" as it is for a system with a lot of electronic leakage.)

 

[Lucien Nunes]

In theory, before the supply characteristics are changed, the suitability of the protective measures in the installation should be confirmed as suitable for the new supply. I expect this is rarely done for small installations, not least because the impedance of a small generator in the tens of kVA is usually difficult to define, over the ADS tripping time under consideration.

A rule of thumb sometimes applied to synchronous generators in this size range is PFC = 7*FLC but there are too many variables to allow this simple figure to be used to assess the suitability of edge-case ADS design. For example, where in the cycle the fault occurs (DC component contribution); what load is on the machine at the time (prime mover instantaneous power availability); load angle (demagnetisation effect of fault current); AVR characteristics etc. Due to the relatively high R/Z of a machine of this size, the pf of the fault current is unfavourable and it will both demagnetise the machine and mechanically arrest it, resulting in a very broad range of current/time curves.

For all the above reasons, it is normal to employ overall RCD protection in this scenario.

 

[Cookie]

Many generators have RCD on the output for this sort of reason, as well as protection for socket outlets against shock. Typically don't have the PFC in reality to trip things (even if they measure as "stiff" due to electronic regulation on they usual style of impedance meter).

(We are looking at a generator in due course, probably 10-20kVA 3-phase and probably will ask the supplier for a delay 100mA RCD and hard-wired terminals instead of socket and 30mA "instant" as it is for a system with a lot of electronic leakage.)

Is it required by BS7671?

Me personally I really do not want leakage current to trip an RCD supplying power to fire alarms, emergency lighting or even medical equipment.

 

[Cookie]

In theory, before the supply characteristics are changed, the suitability of the protective measures in the installation should be confirmed as suitable for the new supply. I expect this is rarely done for small installations, not least because the impedance of a small generator in the tens of kVA is usually difficult to define, over the ADS tripping time under consideration.

A rule of thumb sometimes applied to synchronous generators in this size range is PFC = 7*FLC but there are too many variables to allow this simple figure to be used to assess the suitability of edge-case ADS design. For example, where in the cycle the fault occurs (DC component contribution); what load is on the machine at the time (prime mover instantaneous power availability); load angle (demagnetisation effect of fault current); AVR characteristics etc. Due to the relatively high R/Z of a machine of this size, the pf of the fault current is unfavourable and it will both demagnetise the machine and mechanically arrest it, resulting in a very broad range of current/time curves.

For all the above reasons, it is normal to employ overall RCD protection in this scenario.

Right, but who does this in an emergency? Supermarket, data center, school turned into a shelter- during a storm the owner asks that a gen be wheeled and hooked up to the mains supply

Would you know of any typical values for units in the 10 to 100kw range? What about 500-1000kw?

From what I've read a single phase to ground fault is capable of producing 10x the FLC for one second, then that value drops to 3x for 9 seconds thereafter. But this is really thin on my part.

 

[Strima]

Most back up supplies that are brought in are normally only for essential circuits, such as Supermarkets should only have support for fridge's and freezers, basic lighting circuits etc. These curcuits should be designed to accommodate this.

Data centres tend to use a UPS to support essential circuits until the generator comes on full load from a dark start, roughly 2 minutes for most places. The loads are fed through the UPS which normally limits any fault currents as part of it design.

 

[Cookie]

Most back up supplies that are brought in are normally only for essential circuits, such as Supermarkets should only have support for fridge's and freezers, basic lighting circuits etc. These curcuits should be designed to accommodate this.

Data centres tend to use a UPS to support essential circuits until the generator comes on full load from a dark start, roughly 2 minutes for most places. The loads are fed through the UPS which normally limits any fault currents as part of it design.

Right, but you've never seen a mobile gen come in or a permanently installed one put in place latter?

 

[Strima]

Right, but you've never seen a mobile gen come in or a permanently installed one put in place latter?

Normally the on site generators are short term, I know data centres work on a four hour window. If the outage exceeds this then external generators are brought in, these plug into the site generator point which would then be running the essential circuits as I mentioned earlier.

As Lucien pointed out generators can be problematic and the installation should be assessed before they're connected.

 

[Cookie]

Normally the on site generators are short term, I know data centres work on a four hour window. If the outage exceeds this then external generators are brought in, these plug into the site generator point which would then be running the essential circuits as I mentioned earlier.

As Lucien pointed out generators can be problematic and the installation should be assessed before they're connected.

Understood, but where I'm from no one really assesses. Certainly not a home owner who gets a 10 or 15kw portable unit hooked up to his home after a storm.

Commercial places will call a gen rental company and they will hook up to the main switch board without going through every circuit.

 

[DefyG]

but where I'm from no one really assesses.

They do but you may be right in that home owners don't!