Dear Mr Liew Modern industrial products often use computer and switch-mode power techniques which are a powerful source of RF interference, and to combat this their manufacturers usually fit RF filters to their AC supplies. Sometimes the filters are fitted to protect them from interference carried by the AC supply.
The problem with these RF filters is that they generally have capacitors fitted between their AC phases and earth, and this provides a leakage path for current at the supply frequency (50 or 60 Hz). For example, at 50Hz a 4.7nF capacitor in an RF filter on 230 Vac will leak 0.34mA, and a 100nF capacitor will leak 6.8mA. This is not a great deal of current, but when you have a number of equipments all fitted with RF filters and running from the same supply it can easily add up to enough to trip an earth-leakage device. Also, when you have all these capacitors from phases to earth, a mains voltage surge creates a similar surge in their earth leakage current. People have been having similar problem with earth leakage in systems comprising many items of equipment for many years! To prevent "nuisance" tripping of earth-leakage breakers The usual fix is not to use an earth-leakage breaker at all! Or (more expensively) if you wish to preserve the safety features of the earth leakage breaker - fit a 1:1 isolating transformer in the AC supply between the earth leakage device and the offending equipment(s). The transformer needs to be suitably rated for the load power, and should also be over-rated to cope with the harmonic currents drawn by the load (especially significant in variable-speed motor drive applications). Power transformer manufacturers should be able to advise on rating for load power and harmonics. It is easy enough to find out in advance if a combination of equipments will cause an earth leakage problem - just add up their earth leakages, using either the figures specified by their manufacturers or measuring them with an ammeter in series with their earh lead, and comparing the total earth current with the rating of the earth leakage breaker. It is not a good idea to allow the total earth leakage current to exceed 40% of the breaker's trip setting or you will get a lot of "nuisance" tripping. You may also find it advantageous to use earth-leakage breakers that are not very fast acting and will allow momentary transient currents to occur. Whichever route you take, the total earth leakages can be high enough to cause a very real safety hazard for electricians who don't take care to isolate equipment power before altering wiring. Since many RF filters are fitted BEFORE the on/off switches in equipment they continue to create earth leakage currents even when the equipment is switched off! Consequently it is very important for electricians to always isolate all the phases and neutrals of the AC supplies (i.e. switch them off at the distribution switch-box) to the entire area of equipment they are intending to work on. Since some filters may not be fitted with discharge resistors, it is also a good idea to short all the phases, neutral, and earth together (after switching the power off with a switch that isolates all the phases and the neutral!) to discharge any charged-up filter capacitors. I hope this helps. All the very best! Keith Armstrong Partner, Cherry Clough Consultants Liew, Shih Perng wrote: > > Hi All, > > I have some different machines in one production line sharing a same > power source, namely IX testers, ovens etc. I encountered a problem > where some of the ovens tripped-off intermittently. The earth-leakage > current was found to be excessive and causes the earth-leakage circuit > breaker to trip. To find a short-term solution, the affected ovens > were rewired from a new power source and the metal structures were > insulated. The problem seems to be solved...however, I am not so > convinced. > > Please advise if it is possible and how to find out before hand which > combination of machines will work together or could share a same > source without experiencing inteference. Can simulation method be > applide in this case? > > Bear with me if this doesn't make any sense, I am struggling to > understand the terms passed down to me at the moment. > > Thanks in advance. > > rgds, > sp
