http://en.wikipedia.org/wiki/Varistorhttp://en.wikipedia.org/wiki/Varistor

RS Part number http://rswww.com/cgi-bin/bv/rswww/searchBrowseAction.do?D=varistor&Nr=AND%28avl%3auk %2csearchDiscon_uk%3aN%29&Ntk=I18NAll&Nty=1&Ntt=va ristor&Dx=mode%20matchpartial&Ntx=mode%20matchpart ial&N=0&name=SiteStandard&forwardingPage=line&R=22 68040&callingPage=/jsp/search/search.jsp&BV_SessionID=@@@@0672139044.1184427126@ @@@&BV_EngineID=ccdeaddlhgkgdlhcefeceeldgkidhgi.0& cacheID=uknetscape226-8040 is the sort of thing you need I think.