Further to Richard Garbett's comments - I understand that the build-up of leakage currents in the earth can pose hazards to electricians when they are working on wiring systems if they don't take care to isolate the phases and neutrals of the entire section they are working on. This is because (as I said) filters may be permanently in circuit and not isolated by the on/off switch on their units, so still leaking.
What can happen is that the electrician decides to remove an earth wire from a unit, maybe to reroute it or something, having first checked that the unit is switched off. The leakage current is still flowing so when he disconnects the earth wire he can get a shock. In humid environments even 0.25mA current is enough to create a "touch response" i.e. a muscular twitch which could cause him to cut or bruise his hand, bang his head, etc. If he is working up a ladder he could fall off it. If he is working near to live busbars he could accidentally touch the busbars. Most injuries due to electric shock are actually caused by the effects of falling over! On the subject of stored energy in filters - OK, my original advice to short out the phases and neutrals can result in big sparks which can cause damage and give the electrician a fright, making him jump, a bad idea. Maybe instead he should use his voltmeter on AC and DC to check that there actually are no voltages between any of the phases or neutral and earth. I understand that this practice is required by UK Health and Safety at Work Regulations for electricians working on supply distribution systems anyway, to check that the system really has been isolated before starting work, although they would not include the DC test. If he finds a DC voltage of more than 10V or so: either leave it for a while until it has self-discharged (e.g. through the meter's resistance), or else discharge it to earth using a resistor (1k should discharge 230/415V filter capacitors quickly enough without causing sparks or getting too hot). On the topic of discharge resistors in filters: I believe that the safety authorities are quite right to insist on bleed resistors. Ever pulled a plug from the wall and then got a shock from its exposed pins? Try it sometime with 100nF live-neutral - even at this low capacitance value (motor drives can use several microfarads) it can give a nasty jolt. Now imagine getting this surprise while up a ladder or in a confined maintenance space full of sharp objects. Maybe its not so bad on 110V supplies, but its no joke on 230V or 415V. Without bleed resistors some filters can take many minutes to discharge. I think that it is a perfectly reasonable requirement for filter manufacturers to spend a few extra cents/pennies on fitting safety-approved bleed resistors, and it is a sad reflection on our times if this has to be enforced by safety agencies instead of being recognised as good professional engineering practice. My own expertise includes designing products so that they don't need to rely on heavy duty filters, saving money and weight and also reducing many of the leakage and stored charge safety problems. Bolting large filters on to a less than-optimally-designed product is not a good commercial proposition. All the best! Keith Armstrong Partner, Cherry Clough Consultants Richard Garbett (MPECD) wrote: > > Some good ideas are stated in the below comments, but also some very > dangerous ones are mentioned to. > > The use of EMC filters sounds the ideal solution for this problem, the > moving of the machines onto different supplies has just limited the earth > leakage to a lower level. Problems may start to occur once all the machines > are all working at a full working load, you may find that the earth leakage > trip may start tripping yet again. EMC filters of today are made to > customers requirements, so limiting the amount of Earth leakage that a > filter produces can be limited to a minimum, or even in some circumstances > eliminated all together and solve your problem of "blowing earth leakage > trips". > > As for the Safety of the electricians there should be no real concern. If a > electrician is careful enough not to cause him/her self any harm in dealing > with the electricity in the first place, the introduction of an EMC filter > is causing no more danger to the electrician. The only way that an > electrician may cause him/her self any harm is by shorting out the phase on > the filter, not a good idea this may cause damage to the electrician and to > the filter itself. > > Also it would also be nice to find a safety agency that would allow a filter > to be manufactured and approved without the inclusion of bleed resistors > within the circuitry. > > So my recommendation would be get into contact with a EMC filtration company > and seek advice, there are many out there. > > > -----Original Message----- > > From: Keith Armstrong [SMTP:[email protected]] > > Sent: 23 June 1998 17:30 > > To: Liew, Shih Perng > > Cc: [email protected] > > Subject: Re: EMC in motors and drives > > > > 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
