T.O. 33B-1-13-143.1.9.4 PortableEquipment.CAUTIONContact prods shall not be used on aerospace components or parts, and SHALL belabeled: NOT FOR AIRCRAFT USE.Portable MT units are manufactured in a variety of sizes, shapes, voltages, and current outputs. Portable equipmentoperates on the same principle as stationary equipment. However, the compactness allows areas to be inspected wherelarger equipment may prohibit access. Portable equipment is usually operated on 110 or 220 volt AC and is ratedbetween 200 and 1000 amperes. Portable equipment can be either AC, or a combination of AC and halfwave DC.Portable equipment is suitable for examining small areas in large components where suspected cracks may be found.For example, critical engine mount fittings and landing gear assemblies, which are difficult to inspect in stationaryunits, can be examined quickly with minimum disturbance and with attention concentrated on points most subject tocracking. Portableequipment can be moved to large items in need of magnetic particle testing and inspections canoften be performed without disassembly.3.1.9.5 ContactProds.CAUTIONWhen parts are being magnetized by the use of spring loaded contact clamps togenerate circular magnetization, the contact clamps shall not conduct more than800 amperes.When a part is too large to fit into a stationary unit, or if only mobile or portable equipment is available, then the part,or areas of the part, can be magnetized using cables and two hand-held prods. The current passing between the twocontact prods creates a circular field. Great care must be used to prevent local overheating, arcing, or burning thesurface being inspected, particularly on high-carbon or alloy materials where hard spots or cracks could be produced.3.1.9.6 ContactClamps.Contact clamps can be used with cables instead of contact prods, particularly when the parts are relatively small indiameter. Parts, like engine mounts and tubular structures, are inspected by positioning the clamps so that currentpasses through the area under inspection. Care must be used to avoid burning of the part under the contact clamps.Burning and heating may be caused by dirty contacts, insufficient contact clamp pressure, or excessive currents.Cracks may be produced as a result of the transient heating.3.1.9.7 HandprobeOrYoke.For occasional inspections of small parts and localized inspection of large parts, magnetic hand probes or yokes areoften adequate and easy to use. Probes or yokes are able to put a strong magnetic field into that portion of the part thatlies between the poles of the probe or yoke. Hand probes or yokes can be excited by either AC or DC electricalcurrents, or they can be a permanent magnet.3.1.9.8 ElectromagneticProbeorYoke.Electromagnetic probes or yokes are U-shaped cores of soft iron with a coil wound around the base of the U (see Figure3-18). When alternating current or rectified alternating current is passing through the coil, the core is magnetized.This produces an electromagnetic probe or yoke that functions similarly to a permanent horseshoe magnet. A probe oryoke may be used to induce only a longitudinal field in a part. No electrical current passes through the part.
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