This place really makes me laugh at times
As has been stated, I.T equipment use mains filters/suppressors that are designed to leak current to earth (through capacitors/resistors) - Hence each piece of kit may "leak" a mA or two (hypothetically - varies depending on equipment) - Multiply by 50 or 100 units like in an office and you will exceed the trip rating of a 30mA RCD.
Most offices in the past weren't RCD'ed - but going forward there is a requirement to use one (cable protection in walls (unless under floor fed)) - hence office rings may need new smaller rings to prevent leakage currents being too high.
The use of twin earth terminals in 13A sockets does not reduce residuals - it is to comply with "high integrity CPC circuits" - its all in the name - "high integrity". Which means that the CPC is key to the circuit, even more than normal, hence you terminate the ring CPC into two separate terminals, in case one comes loose, or broken.
D.
Indeed so DD. That is my understanding..
Additionally..
The equipment will still leak the amount of current whether 1 or 2 CPC connections.
Consider a classroom or large office say 35 screens & base units each screen & Base leaks 0.33ma, 35x2x0.33 =23ma+ you only need a few more bits plugged in or a bit of a surge or a sensitive RCD and you have a nuisance trip!
Someone one an earlier post said that a larger earth was required.. the regs say with a ring it is quite acceptable to use two connected cpc terminations of the same CSA.
If it was a radial.. an additional CPC back to thew CU is needed, because this is outside of another cable it can require additional mechanical protection.. thus it bigger!
Also there can be problems if the IT equipment doesn't have a good earth as its own power surge suppression, which smooths out the natural fluctuations in the mains supply, and may not work effectively! which is why it needs a High Integrity earth NOT a Clean Earth!
CLEAN EARTHS!
The concept of clean earths goes back to earlier days of telecommunications and data communications over traditional twisted pair telephone cables..
Because some data transmissions used signalling with reference to an earth point, e.g... between two factory sites.. or around large factories... (e.g. car plants etc..)
If the earth had a lot of noise, (induced voltages, due to factory plant etc..) this could cause errors on the data transmission. (we once had a 4pair twisted cable with 16v AC induced onto it, our RS485 line drivers were supposed to signal using 12v!
) Screening & tying down to a clean earth did the trick!
As transmission was at a slower rate with less fancy error correction, a good quality earth (clean earth) was often requested to maximise transmission reliability.
Hoverer this is less of an issue with faster communications, Co-Ax cables, more efficient error correction..... AND..... Fibre Optic transmisions..... light is NOT referenced to earth! :|
(clean earths could on occasions cause more problems, due to earth pd between various site locations)
The other aspect of Earth's & IT, is surge protection to avoid any transient spikes from lightning induced voltages up the CPC damaging the IT equipment.
All interrelated but all different:-
Clean Earth... Signalling & Comms'
Surge Protection... lightning & noise
High Integritey... Ensure a path to allow leakage current to pass.
These I understand are what is know as Functional Earths, as compared to Protective Earths, although obviously the High integrity earth will perform both functional & protective.
thats my understanding of the issue anyway?
:^OGuiness Drink
