当dropdownlist变化时候一定要设置autopostback=true 才能激活出发事件

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<td class="style10">

Fault Condition1:

</td>

<td>

<asp:ScriptManager runat="server">

</asp:ScriptManager>

<asp:UpdatePanel ID="UpdatePanel6" runat="server">

<ContentTemplate>

<asp:DropDownList ID="Fault1_DropDownList" AutoPostBack=true runat="server" Width="254px" OnSelectedIndexChanged="fault1changed">

</asp:DropDownList>

</ContentTemplate>

</asp:UpdatePanel>

</td>

</tr>

<tr>

<td class="style7">

Fault1 Severity:

</td>

<td class="style3">

<asp:UpdatePanel ID="UpdatePanel1" runat="server">

<ContentTemplate>

<asp:DropDownList ID="Severity_DropDownList" runat="server" AutoPostBack="true" OnSelectedIndexChanged="severityChanged"

Width="133px">

<asp:ListItem Text="NORMAL" Value="1">

</asp:ListItem>

<asp:ListItem Text="ABNORMAL" Value="2">

</asp:ListItem>

<asp:ListItem Text="MODERATE" Value="3">

</asp:ListItem>

<asp:ListItem Text="SEVERE" Value="4">

</asp:ListItem>

<asp:ListItem Text="NOT TESTED" Value="5">

</asp:ListItem>

</asp:DropDownList>

</ContentTemplate>

<Triggers>

<asp:AsyncPostBackTrigger ControlID="Fault1_DropDownList" />

</Triggers>

</asp:UpdatePanel>

</td>

</tr>