Application Note Earth Leakage Toroids - Blue Electronics

1 ELCB003 RV 3 22/11/99 CRN: 4473 AMPCONTROL CSM Pty Limited ACN 000 770 141250 Macquarie Road Warners Bay NSW 2282 Phone: (02) 4956 5899 Fax: (02) 4956 5985technicalApplication NoteEarth Leakage Toroids1. IntroductionToroids (current transformers) are not ideal devicesand if correct procedures are not followed duringinstallation nuisance tripping can result. If, forexample, we consider a single-phase Earth leakagesystem where active and neutral pass through a toroidthen at all times currents in the two wires are equaland opposite so that the net current through the toroidis zero.

2 An ideal current transformer would have all ofthe flux from each wire contained in the core and sowould accurately add the opposing fluxes to get a netresult of zero. A real current transformer has leakagefluxes . That is, a very small proportion of the totalflux from each cable is not contained in the core but inthe space outside it and as a result it may link someturns but not others, depending on the positioning ofthe cables. The effect of this is that a small output maybe obtained from the toroid where none would arise ifthe device were size of the error may vary from Toroids of thesame type because of slight differences in the core andthe symmetry of the winding.

3 Problems caused in thisway increase as the toroid size increases, as currentsincrease and symmetry decreases. Nuisance trippingtends to occur when the total current rises, such aswhen a large motor is started. The followingguidelines would help to avoid such Selection Guidelines1. Select the smallest internal diameter toroid, whichwill allow the cables to fit through. Avoid verylarge Toroids (200m) or Toroids with Only use approved Toroids specified byAmpcontrol as these have been designed tominimise the Installation Guidelines1.

4 Keep cables as close to the centre of the toroid aspossible. Do not tie them to one side of the to aim at symmetry2. Do not bring the cables back past the toroid withinone diameter of the toroid. Trying to cram cablesinto a small space reduces symmetry and maylead to problems, which are difficult to Avoid placing the toroid near any device, whichproduces magnetic fields. This includes bus bars,transformers or other cables. Try to maintainseveral toroid diameters Many small cables tend to be worse than say threelarge ones. Try to position the toroid in the circuitwith this in Toroids used for core balance Earth leakageprotection cannot have bus bars passed throughthe Where high currents are experienced during startup an Ampcontrol High Performance Toroid mustbe used to prevent nuisance When the toroid is used on non- Earth fault limitedsystems an Ampcontrol Snubber should beconnected across the toroid terminals to preventdamage to the relay.

5 See Application NoteELCB006 for further information and To prevent possible nuisance tripping it issuggested that the conductor screen of the earthleakage toroid should be earthed one end only, therelay end. If both ends are earthed the possibilityexists for the shield to become an Earth loop,having finite resistance and injecting noise intothe toroid Toroid Installation on ArmouredCableIt is important that correct installation procedures arefollowed when installing a toroid over an armouredcable. The cone and mounting bolts must be insulatedfrom any earthed metal enclosures.

6 The lead is earthedto the cone and then passed through the toroid andconnected to Earth . If this procedure is not followed anearth fault may not be detected. See Figure 1, Page Application NOTET oroid Installation on Armoured CableFigure 1